[test-improver] Improve tests for tracing package

[github-actions]

Test Improvements: tracing package

Files Analyzed

• Test Files: internal/tracing/parse_headers_test.go, internal/tracing/http_test.go
• Package: internal/tracing

Improvements Made

1. Increased Coverage

• ✅ Added TestParseOTLPHeadersWithDecoder_InvalidPercentEncoding — exercises the url.PathUnescape error branch in parseOTLPHeadersWithDecoder (previously uncovered). Verifies the raw value is preserved when percent-decoding fails (e.g. %GZ is not valid percent-encoding).

• ✅ Added TestParseOTLPHeadersWithDecoder_ValidPercentEncoding — documents and verifies that well-formed percent-encoded values are decoded when decodeValues=true.

• ✅ Added TestParseOTLPHeadersWithDecoder_NoDecoding — documents and verifies that percent-encoded values are preserved as-is when decodeValues=false.

• ✅ Added TestWrapHTTPHandler_PatternMethodMismatch_ClearsRoute — exercises the else { route = "" } branch in WrapHTTPHandler (previously uncovered). This path is triggered when r.Pattern contains a method prefix that differs from the actual request method.

• Previous Coverage: 96.4%

• New Coverage: 97.6%

• Improvement: +1.2%

2. Better Testing Patterns

• ✅ New tests use table-driven style where multiple variants are needed
• ✅ Tests include descriptive names following Test<Func>_<Scenario> convention
• ✅ Tests are independent with no side effects

Test Execution

All tests pass:

ok  	github.com/github/gh-aw-mcpg/internal/tracing	0.017s	coverage: 97.6% of statements

All other Go tests continue to pass (make test-all).

Why These Changes?

The internal/tracing package had two meaningful uncovered branches:

1. parseOTLPHeadersWithDecoder — The url.PathUnescape error path was untested. This is a real defensive code path that protects against invalid percent-encoded OTLP header values arriving via the OTEL_EXPORTER_OTLP_HEADERS environment variable.

2. WrapHTTPHandler — The route = "" else-branch when the pattern's HTTP method doesn't match the request method was untested. While this rarely occurs in normal mux routing, it is a defensive code path worth covering.

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