Add QuoteModifier ported from Respect\Validation

Port QuoteModifier from Validation project to StringFormatter following
the modifier pattern. Adapts validation logic to transformation logic
using Stringifier Quoter instead of Quoted placeholder.

- Implements Chain of Responsibility pattern with proper delegation
- Uses CodeQuoter for consistent string escaping behavior
- Includes comprehensive test suite with TestingQuoter helper
- Updates AGENTS.md with modifier development guidelines

Author: henriquemoody

AGENTS.md

@@ -29,6 +29,27 @@ When creating new formatters:
 
 All formatters must implement the `Respect\StringFormatter\Formatter` interface.
 
+## Modifier Development
+
+When creating new modifiers:
+
+1. **Follow Chain of Responsibility pattern**: Check pipe value and delegate to next modifier
+2. **Use template structure**: Similar to `src/Modifier/QuoteModifier.php` 
+3. **Test with TestingModifier**: Located in `tests/Helper/TestingModifier.php`
+4. **Handle type checking**: Always check input types before processing
+5. **Return string values**: Modifiers must return strings
+6. **Use Stringifier Quoter**: For string operations, inject `\Respect\Stringifier\Quoter` with CodeQuoter as default
+
+All modifiers must implement the `Respect\StringFormatter\Modifier` interface.
+
+## Testing Guidelines
+
+1. **Avoid PHPUnit mocks**: Create custom test implementations instead of using createMock()
+2. **Use custom test quoter**: Follow pattern in `tests/Helper/TestingQuoter.php`
+3. **Test contracts not implementations**: Verify interactions without depending on specific behavior
+4. **Make test properties public**: When using anonymous classes to access test state
+5. **Verify method calls**: Track whether methods were called and with what parameters
+
 ## Commit Guidelines
 
 Follow the detailed rules in `docs/contributing/commit-guidelines.md`:

docs/modifiers/Modifiers.md

@@ -33,12 +33,13 @@ Modifiers form a chain where each modifier can:
 The default modifier chain is:
 
 ```
-RawModifier -> ListModifier -> StringifyModifier
+RawModifier -> ListModifier -> QuoteModifier -> StringifyModifier
 ```
 
 - `StringifyModifier` is always the last modifier, using the stringifier to convert values to strings
 - `RawModifier` processes scalar values without stringifier formatting
 - `ListModifier` formats arrays as human-readable lists with "and" or "or" conjunctions
+- `QuoteModifier` quotes string values using the stringifier quoter
 
 ## Syntax Rules
 
@@ -72,6 +73,7 @@ For non-scalar values with `|raw`, the modifier falls back to the next modifier
 - **[RawModifier](RawModifier.md)** - Outputs scalar values directly without stringifier formatting
 - **[StringifyModifier](StringifyModifier.md)** - Default modifier that uses stringifier for all values
 - **[ListModifier](ListModifier.md)** - Formats arrays as human-readable lists with conjunctions
+- **[QuoteModifier](QuoteModifier.md)** - Quotes string values using a stringifier quoter
 
 ## Creating Custom Modifiers
 
@@ -93,4 +95,4 @@ $formatter = new PlaceholderFormatter(
 );
 ```
 
-If no modifier is provided, the formatter uses a default `RawModifier` with `ListModifier` and `StringifyModifier`.
+If no modifier is provided, the formatter uses a default `RawModifier` with `ListModifier`, `QuoteModifier` and `StringifyModifier`.

docs/modifiers/QuoteModifier.md

@@ -0,0 +1,112 @@
+# QuoteModifier
+
+The `QuoteModifier` adds proper quoting to string values using a configurable quoter.
+
+## Purpose
+
+The `|quote` modifier applies string quoting to values, handling escape sequences and special characters according to the configured quoter implementation.
+
+## Behavior
+
+The modifier only processes string values. For non-string values, it passes them to the next modifier in the chain.
+
+### Quoting Behavior
+
+- Strings are quoted using the configured `Quoter` instance (default: `CodeQuoter` with 256 character limit)
+- Non-string values are passed to the next modifier unchanged
+- Handles escape sequences and special characters according to the quoter implementation
+
+## Usage
+
+```php
+use Respect\StringFormatter\PlaceholderFormatter;
+use Respect\StringFormatter\Modifier\QuoteModifier;
+use Respect\StringFormatter\Modifier\StringifyModifier;
+
+$formatter = new PlaceholderFormatter([
+    'name' => 'John "Johnny" Doe',
+    'path' => 'C:\Users\Files\My Document.txt',
+    'status' => 'It\'s working',
+    'age' => 30,
+    'data' => ['x' => 1]
+]);
+
+echo $formatter->format('{{name|quote}}');
+// Outputs: '"John \"Johnny\" Doe"'
+
+echo $formatter->format('{{status|quote}}');
+// Outputs: '"It\'s working"'
+
+echo $formatter->format('{{age|quote}}');
+// Outputs: "30" (passed to StringifyModifier, no quoting applied)
+```
+
+## Examples
+
+### Basic Quoting
+
+| Parameters | Template | Output |
+| ---------- | -------- | ------ |
+| `['name' => 'John']` | `"{{name|quote}}"` | `"\"John\""` |
+| `['text' => 'Hello "World"']` | `"{{text|quote}}"` | `"\"Hello \\\"World\\\"\""` |
+| `['path' => 'C:\temp']` | `"{{path|quote}}"` | `"\"C:\\temp\""` |
+
+### Escape Sequences
+
+| Parameters | Template | Output |
+| ---------- | -------- | ------ |
+| `['text' => "Line1\nLine2"]` | `"{{text|quote}}"` | `"\"Line1\\nLine2\""` |
+| `['text' => "Tab\tSeparated"]` | `"{{text|quote}}"` | `"\"Tab\\tSeparated\""` |
+| `['text' => "Back\\Slash"]` | `"{{text|quote}}"` | `"\"Back\\\\Slash\""` |
+
+### Non-string Values
+
+| Parameters | Template | Output |
+| ---------- | -------- | ------ |
+| `['age' => 30]` | `"{{age|quote}}"` | `"30"` |
+| `['active' => true]` | `"{{active|quote}}"` | `"true"` |
+| `['items' => ['a', 'b']]` | `"{{items|quote}}"` | `["a","b"]` |
+
+## Custom Quoter
+
+You can create a `QuoteModifier` with a custom quoter:
+
+```php
+use Respect\StringFormatter\PlaceholderFormatter;
+use Respect\StringFormatter\Modifier\QuoteModifier;
+use Respect\StringFormatter\Modifier\RawModifier;
+use Respect\StringFormatter\Modifier\StringifyModifier;
+use Respect\Stringifier\Quoters\CodeQuoter;
+
+// Custom quoter with different character limit
+$customQuoter = new CodeQuoter(512);
+$quoteModifier = new QuoteModifier($rawModifier, $customQuoter);
+
+$formatter = new PlaceholderFormatter(['text' => $value], $quoteModifier);
+```
+
+## Implementation Notes
+
+The `QuoteModifier`:
+
+1. Checks if the pipe value is `"quote"` and the input value is a string
+2. If both conditions are met, applies the configured quoter to the string
+3. Otherwise, passes the value to the next modifier in the chain
+4. Uses `CodeQuoter` by default with a 256 character limit
+
+## Common Use Cases
+
+1. **CSV output** - Properly quote fields containing special characters
+2. **Code generation** - Quote string literals in generated code
+3. **Configuration files** - Ensure string values are properly escaped
+4. **Logging** - Prepare string values for logging with proper escaping
+
+## Integration
+
+`QuoteModifier` is typically positioned early in the chain:
+
+```php
+QuoteModifier -> RawModifier -> StringifyModifier
+```
+
+This ensures string quoting is applied before other processing, while still allowing raw output and stringification fallbacks.

src/Modifier/QuoteModifier.php

@@ -0,0 +1,29 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Respect\StringFormatter\Modifier;
+
+use Respect\StringFormatter\Modifier;
+use Respect\Stringifier\Quoter;
+use Respect\Stringifier\Quoters\CodeQuoter;
+
+use function is_string;
+
+final readonly class QuoteModifier implements Modifier
+{
+    public function __construct(
+        private Modifier $nextModifier,
+        private Quoter $quoter = new CodeQuoter(256),
+    ) {
+    }
+
+    public function modify(mixed $value, string|null $pipe): string
+    {
+        if ($pipe !== 'quote' || !is_string($value)) {
+            return $this->nextModifier->modify($value, $pipe);
+        }
+
+        return $this->quoter->quote($value, 0);
+    }
+}

tests/Helper/TestingQuoter.php

@@ -0,0 +1,36 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Respect\StringFormatter\Test\Helper;
+
+use Respect\Stringifier\Quoter;
+
+final class TestingQuoter implements Quoter
+{
+    public string $lastQuotedValue = '';
+
+    public int $lastQuotedDepth = -1;
+
+    public bool $quoteWasCalled = false;
+
+    public function __construct(public readonly string $returnValue = 'test-output')
+    {
+    }
+
+    public function quote(string $string, int $depth): string
+    {
+        $this->quoteWasCalled = true;
+        $this->lastQuotedValue = $string;
+        $this->lastQuotedDepth = $depth;
+
+        return $this->returnValue;
+    }
+
+    public function reset(): void
+    {
+        $this->lastQuotedValue = '';
+        $this->lastQuotedDepth = -1;
+        $this->quoteWasCalled = false;
+    }
+}