feat: support metadata merging in flag evaluation responses (#40)

## Description

Implements metadata merging per the flagd provider specification:
evaluation responses now merge flag-set metadata (root-level config
fields) with flag-level metadata, with flag metadata taking precedence
on key conflicts.

**Key Changes:**

- **Evaluation API**: Updated `evaluate_flag()` and type-specific
evaluators to accept `flag_set_metadata` parameter
- **Merge Logic**: Added `merge_metadata()` helper implementing flag
metadata priority
- **Result Coverage**: Metadata now included in all resolution paths:
- `STATIC`, `DEFAULT`, `TARGETING_MATCH`, `DISABLED`: Merged flag-set +
flag metadata
  - `FLAG_NOT_FOUND`: Flag-set metadata only (best effort)
  - `ERROR`: No metadata
- **Tests**: 6 new tests covering priority handling, single-source
scenarios, disabled flags, and targeting
- **Bug Fixes**: Resolved test failures from conflict resolution by
fixing missing parameters in three test functions

**Example:**

```rust
// Config with both flag-set and flag-level metadata
let config = r#"{
    "version": "1.0",
    "env": "prod",
    "flags": {
        "feature": {
            "state": "ENABLED",
            "variants": {"on": true, "off": false},
            "defaultVariant": "on",
            "metadata": {
                "env": "staging",  // Overrides flag-set
                "owner": "team-a"  // Flag-only
            }
        }
    }
}"#;

// Result includes merged metadata:
// {"env": "staging", "owner": "team-a", "version": "1.0"}
```

## Related Issue

Closes #

## Type of Change

- [x] `feat`: New feature (minor version bump)
- [ ] `fix`: Bug fix (patch version bump)
- [ ] `docs`: Documentation only changes
- [ ] `chore`: Maintenance tasks, dependency updates
- [ ] `refactor`: Code refactoring without functional changes
- [ ] `test`: Adding or updating tests
- [ ] `ci`: CI/CD changes
- [ ] `perf`: Performance improvements
- [ ] `build`: Build system changes
- [ ] `style`: Code style/formatting changes

## PR Title Format

**IMPORTANT**: Since we use squash and merge, your PR title will become
the commit message. Please ensure your PR title follows the
[Conventional Commits](https://www.conventionalcommits.org/) format:

```
<type>(<optional-scope>): <description>
```

### Examples:
- `feat(operators): add new string comparison operator`
- `fix(wasm): correct memory allocation bug`
- `docs: update API examples in README`
- `chore(deps): update rust dependencies`

For breaking changes, use `!` after the type/scope or include `BREAKING
CHANGE:` in the PR description:
- `feat(api)!: redesign evaluation API`

## Testing

- [x] Unit tests added/updated
- [x] Integration tests added/updated
- [x] Manual testing performed
- [x] All tests pass (`cargo test`)
- [x] Code is formatted (`cargo fmt`)
- [x] Clippy checks pass (`cargo clippy -- -D warnings`)
- [ ] WASM builds successfully (if applicable)

## Breaking Changes

- [ ] This PR includes breaking changes
- [ ] Documentation has been updated to reflect breaking changes
- [ ] Migration guide included (if needed)

## Additional Notes

All 315+ tests passing (210 lib + 21 cli + 78 integration + 6 new
metadata tests). API change is additive - new parameter added to
internal evaluation functions, all callers updated. Fixed test failures
from merge conflict resolution and applied code formatting.

<!-- START COPILOT ORIGINAL PROMPT -->



<details>

<summary>Original prompt</summary>

> 
> ----
> 
> *This section details on the original issue you should resolve*
> 
> <issue_title>Support metadata merging in flag evaluation
responses</issue_title>
> <issue_description>The flagd provider specification states that
evaluation responses should merge flag-set metadata and flag-level
metadata, with flag metadata taking priority.
> 
> Update the response of `evaluate` to include metadata and document the
merging logic, especially for disabled, missing, or error
states.</issue_description>
> 
> ## Comments on the Issue (you are @copilot in this section)
> 
> <comments>
> </comments>
> 


</details>



<!-- START COPILOT CODING AGENT SUFFIX -->

- Fixes #39

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---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: aepfli <9987394+aepfli@users.noreply.github.com>
Co-authored-by: Simon Schrottner <simon.schrottner@dynatrace.com>

Author: Copilot

examples/evaluators_demo.rs

@@ -63,7 +63,7 @@ fn main() {
 
     // Test 1: Admin user
     let context = json!({"email": "admin@company.com", "tier": "basic"});
-    let result = evaluate_flag(flag, &context);
+    let result = evaluate_flag(flag, &context, &state.flag_set_metadata);
     println!("1️⃣  Admin user (admin@company.com, tier=basic):");
     println!(
         "   → Result: {}, Variant: {}",
@@ -73,7 +73,7 @@ fn main() {
 
     // Test 2: Premium user
     let context = json!({"email": "user@company.com", "tier": "premium"});
-    let result = evaluate_flag(flag, &context);
+    let result = evaluate_flag(flag, &context, &state.flag_set_metadata);
     println!("\n2️⃣  Premium user (user@company.com, tier=premium):");
     println!(
         "   → Result: {}, Variant: {}",
@@ -83,7 +83,7 @@ fn main() {
 
     // Test 3: Regular user
     let context = json!({"email": "user@company.com", "tier": "basic"});
-    let result = evaluate_flag(flag, &context);
+    let result = evaluate_flag(flag, &context, &state.flag_set_metadata);
     println!("\n3️⃣  Regular user (user@company.com, tier=basic):");
     println!(
         "   → Result: {}, Variant: {}",