Replace assert.Nil with assert.NoError for error interfaces (#507)

The Go Fan report identified `assert.Nil(t, err)` usage that should use
`assert.NoError(t, err)`. However, this replacement only applies to
error interface types, not concrete pointer types like
`*ValidationError`.

## Changes

- `internal/difc/difc_test.go`: Changed `assert.Nil(t, err)` →
`assert.NoError(t, err)` for `FormatViolationError()` which returns
`error` interface

## Not Changed

Functions returning `*ValidationError` (concrete type) in
`internal/config/rules/rules_test.go` correctly use `assert.Nil`:
- `PortRange()` 
- `TimeoutPositive()`
- `MountFormat()`

## Rationale

Go's typed nil behavior causes `assert.NoError` to fail with concrete
pointer types. When a function returns `*ValidationError` (not `error`),
a nil return creates a typed nil pointer. Converting this to an `error`
interface produces a non-nil interface containing a nil pointer, causing
`assert.NoError` to incorrectly report an error.

```go
// Correct: concrete pointer type
err := PortRange(8080, "path")  // returns *ValidationError
assert.Nil(t, err)  // Works: checks pointer is nil

// Correct: error interface
err := FormatViolationError(...)  // returns error
assert.NoError(t, err)  // Works: checks interface is nil
```

> [!WARNING]
>
> <details>
> <summary>Firewall rules blocked me from connecting to one or more
addresses (expand for details)</summary>
>
> #### I tried to connect to the following addresses, but was blocked by
firewall rules:
>
> - `nonexistent.local`
> - Triggering command: `/tmp/go-build2303605970/b273/launcher.test
/tmp/go-build2303605970/b273/launcher.test
-test.testlogfile=/tmp/go-build2303605970/b273/testlog.txt
-test.paniconexit0 -test.timeout=10m0s -test.v=true tive\|func
MountFormat\|func FormatViolationError -I tnet/tools/as --gdwarf-5 --64
-o 3328745/b183/_x002.o -uns�� ache/go/1.25.6/x64/src/net
/tmp/go-build2873328745/b104/vet.cfg e` (dns block)
> - `this-host-does-not-exist-12345.com`
> - Triggering command: `/tmp/go-build2303605970/b282/mcp.test
/tmp/go-build2303605970/b282/mcp.test
-test.testlogfile=/tmp/go-build2303605970/b282/testlog.txt
-test.paniconexit0 -test.timeout=10m0s -test.v=true -c=4 -nolocalimports
-importcfg /tmp/go-build1521351387/b260/importcfg -pack
/home/REDACTED/.cache/go-build/bb/bb79d626526a1c93efbd89a7ec059b8e00045cd021dd8bc541e1c4e29a277e6a-d
64/pkg/tool/linux_amd64/compile` (dns block)
>
> If you need me to access, download, or install something from one of
these locations, you can either:
>
> - Configure [Actions setup
steps](https://gh.io/copilot/actions-setup-steps) to set up my
environment, which run before the firewall is enabled
> - Add the appropriate URLs or hosts to the custom allowlist in this
repository's [Copilot coding agent
settings](https://github.com/githubnext/gh-aw-mcpg/settings/copilot/coding_agent)
(admins only)
>
> </details>

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<details>

<summary>Original prompt</summary>


----

*This section details on the original issue you should resolve*

<issue_title>[go-fan] Go Module Review: stretchr/testify</issue_title>
<issue_description># 🐹 Go Fan Report: testify

## Module Overview
Testify is Go's most popular testing toolkit with 25,000+ GitHub stars.
It provides comprehensive assertion functions, mocking capabilities, and
testing suite interfaces. The project uses v1.11.1 (latest release from
August 2025, repository updated January 26, 2026).

## Current Usage in gh-aw-mcpg
- **Files**: 67 test files across the entire codebase
- **Import Count**: Extensive usage throughout all test packages
- **Key APIs Used**:
  - ✅ `assert` package - widely used for assertions
- ✅ `require` package - used for critical assertions that stop test
execution
  - ❌ `mock` package - NOT currently used (0 files)
  - ❌ `suite` package - NOT currently used (0 files)

### Usage Patterns Observed
- **Bound Asserters**: Used in ~30% of test files (`assert.New(t)`,
`require.New(t)`)
- **Direct Assertions**: Mixed usage of `assert.X(t, ...)` and bound
asserters
- **Error Handling**: Mostly correct with `require.NoError(t, err)` and
`assert.NoError(t, err)`
- **Specific Assertions**: Good use of semantic methods like `Equal()`,
`Contains()`, `NotNil()`

## Research Findings

### Recent Updates
- **v1.11.1** (August 2025): Fixed regression with stringer interface in
mock argument matching
- **v1.11.0**: Major improvements to mock package, enhanced error
messages
- **Active Maintenance**: Repository was updated January 26, 2026
(today!)

### Best Practices from Maintainers
1. **Use `require` for critical checks** - test can't continue if
assertion fails
2. **Use `assert` for non-critical checks** - gather multiple failures
in one test run
3. **Prefer bound asserters** for tests with multiple assertions
(cleaner syntax)
4. **Use specific assertion methods**:
   - `assert.NoError(t, err)` instead of `assert.Nil(t, err)` for errors
- `assert.Empty(t, slice)` instead of `assert.True(t, len(slice) == 0)`
- `assert.Contains(t, str, substr)` instead of `assert.True(t,
strings.Contains(...))`
5. **Enable testifylint** (via golangci-lint) to catch common mistakes
automatically

### Current Issues & Trends
- **YAML package migration** (Issue #1724):
go-yaml/yaml is archived, testify considering migration
- **v2 Discussion**: Active discussion about v2, but v1 remains stable
with no breaking changes
- **testifylint recommended**: Official recommendation to use linter to
avoid common mistakes

## Improvement Opportunities

### 🏃 Quick Wins

#### 1. Replace `assert.Nil(t, err)` with `assert.NoError(t, err)` - 14
instances
Better error messages specifically for errors and semantic clarity.

**Files affected**:
- `internal/config/rules/rules_test.go` (3 instances)
- `internal/sys/sys_test.go` (3 instances)  
- `internal/logger/common_test.go` (2 instances)
- `internal/mcp/http_connection_test.go` (1 instance)
- `internal/middleware/jqschema_test.go` (1 instance)
- `internal/server/call_tool_result_test.go` (1 instance)
- `internal/config/config_test.go` (2 instances)
- `internal/difc/difc_test.go` (1 instance)

**Example fix**:
```go
// ❌ Before
assert.Nil(t, err, "Unexpected error")

// ✅ After  
assert.NoError(t, err, "Unexpected error")
```

**Impact**: Better error messages when assertions fail, clearer intent

#### 2. Increase Bound Asserter Adoption
Only ~30% of test files use bound asserters. For files with 3+
assertions, bound asserters reduce repetition:

```go
// ❌ Current pattern (verbose)
assert.Equal(t, expected1, actual1)
assert.Equal(t, expected2, actual2)
assert.True(t, condition)

// ✅ Cleaner with bound asserter
assert := assert.New(t)
assert.Equal(expected1, actual1)
assert.Equal(expected2, actual2)
assert.True(condition)
```

**Impact**: More readable tests, less repetition, especially in
table-driven tests

### ✨ Feature Opportunities

#### 1. Consider Mock Package for Interface Testing
The project currently doesn't use testify's `mock` package despite
having interfaces that could benefit:
- Mature and well-integrated with testify
- Alternative: Could use [mockery](https://vektra.github.io/mockery/)
tool for auto-generating mocks
- **Benefit**: Better control over interface behavior in tests, easier
to verify interactions

#### 2. Explore Suite Package for Integration Tests  
The `suite` package could benefit `test/integration/` tests with
setup/teardown:
- Better organization for tests requiring shared state
- Built-in setup/teardown hooks
- **Caveat**: Suite doesn't support parallel tests (known issue
#934)

### 📐 Best Practice Alignment

#### 1. Enable testifylint in golangci-lint
Currently disabled in `.golangci.yml`. AGENTS.md mentions it requires
extensive refactoring, but it would:
- Automatically catch `assert.Nil(err)` → `assert.NoError(err)`
- Suggest better assertion methods
- Enforce consistent patterns

**Recommendation**: Enable gradually...

</details>


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- Fixes #471

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Author: lpcox

internal/difc/difc_test.go

@@ -153,7 +153,7 @@ func TestFormatViolationError(t *testing.T) {
 		resource := NewLabeledResource("test-resource")
 
 		err := FormatViolationError(result, agentSecrecy, agentIntegrity, resource)
-		assert.Nil(t, err, "Expected nil error for allowed access")
+		assert.NoError(t, err, "Expected nil error for allowed access")
 	})
 
 	t.Run("formats secrecy violation error", func(t *testing.T) {