[test] Add tests for logger.SlogHandler.Handle and related functions

[github-actions]

Test Coverage Improvement: slog_adapter.go

Function Analyzed

• Package: internal/logger
• Functions: SlogHandler.Handle, formatSlogValue, NewSlogLoggerWithHandler
• Previous Coverage: 0% (all three functions)
• New Coverage: Handle → 95.2%, formatSlogValue → 100%, NewSlogLoggerWithHandler → 100%
• Complexity: Medium (Handle has multiple branches: 4 log levels, attribute handling, disabled-logger early return)

Why These Functions?

These three functions in internal/logger/slog_adapter.go had 0% coverage because all existing tests guarded execution with:

if os.Getenv("DEBUG") == "" {
    t.Skip("Skipping test: DEBUG environment variable not set")
}
```

In CI environments where `DEBUG` is not set, the tests were always skipped — leaving `Handle`, `formatSlogValue`, and `NewSlogLoggerWithHandler` completely untested. These are important integration points since `SlogHandler` bridges the gh-aw debug logger with Go's standard `slog.Logger` interface.

### Tests Added

- ✅ `TestSlogHandler_Handle_WithDebugEnabled` — All 4 log level prefixes (`[DEBUG]`, `[INFO]`, `[WARN]`, `[ERROR]`) and unknown level fallback (no prefix)
- ✅ `TestSlogHandler_Handle_WhenDisabled` — Early return path when logger is not enabled
- ✅ `TestSlogHandler_Handle_WithAttributes` — String, integer, boolean, float attribute types; multiple attributes; empty message with attributes; no attributes
- ✅ `TestFormatSlogValue` — Both `slog.Value` and non-`slog.Value` inputs (string, int, bool, float, nil)
- ✅ `TestNewSlogLoggerWithHandler_Enabled` — Creates working logger, verifies output with namespace and attributes
- ✅ `TestNewSlogLoggerWithHandler_Disabled` — Verifies no output when logger is disabled
- ✅ `TestNewSlogLoggerWithHandler_MultipleMessages` — Verifies multiple messages through same logger
- ✅ `TestSlogHandler_Handle_AllLevelPrefixes` — Focused table-driven test for all 4 standard slog levels
- ✅ `TestSlogHandler_Handle_NonStringKeyFallback` — Exercises normal attribute key path

### Key Approach

Tests use `t.Setenv("DEBUG", "*")` to **force-enable** the logger rather than skipping. The logger's `enabled` field is computed at construction time via `computeEnabled()`, which reads the `DEBUG` env var — so calling `t.Setenv` before `New()` correctly enables the logger and exercises all code paths.

### Coverage Report

```
Before:
  Handle                  0.0%
  formatSlogValue         0.0%
  NewSlogLoggerWithHandler 0.0%

After:
  Handle                  95.2%  (+95.2%)
  formatSlogValue        100.0%  (+100%)
  NewSlogLoggerWithHandler 100.0%  (+100%)
  
  logger package total:  90.7% → 96.0%  (+5.3%)
```

### Test Execution

```
=== RUN   TestSlogHandler_Handle_WithDebugEnabled
--- PASS: TestSlogHandler_Handle_WithDebugEnabled (0.00s)
=== RUN   TestSlogHandler_Handle_WhenDisabled
--- PASS: TestSlogHandler_Handle_WhenDisabled (0.00s)
=== RUN   TestSlogHandler_Handle_WithAttributes
--- PASS: TestSlogHandler_Handle_WithAttributes (0.00s)
=== RUN   TestFormatSlogValue
--- PASS: TestFormatSlogValue (0.00s)
=== RUN   TestNewSlogLoggerWithHandler_Enabled
--- PASS: TestNewSlogLoggerWithHandler_Enabled (0.00s)
=== RUN   TestNewSlogLoggerWithHandler_Disabled
--- PASS: TestNewSlogLoggerWithHandler_Disabled (0.00s)
=== RUN   TestSlogHandler_Handle_AllLevelPrefixes
--- PASS: TestSlogHandler_Handle_AllLevelPrefixes (0.00s)
ok  	github.com/github/gh-aw-mcpg/internal/logger	0.225s

---

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