test: add edge case coverage for hasHTTPResponseWriterMethods and matchesWriteSignature

[yvonnedevlinrh]

Description

• Severity: Low
• Component: internal/analysis/p1effects.go → hasHTTPResponseWriterMethods
• Related: Bug: WriterOutput false positive: any method named Write is treated as io.Writer #109, Bug: isHTTPResponseWriter uses fragile string comparison instead of types.Implements #132 (discovered during review council analysis)

Summary

hasHTTPResponseWriterMethods requires all three methods (Header, Write, WriteHeader) with correct signatures to classify a type as http.ResponseWriter. This contract is correct but only tested implicitly — there are no edge case tests that verify partial matches are rejected or that near-miss signatures don't pass.

Missing Test Coverage

1. Partial method set (2 of 3 methods)

A type with Write([]byte) (int, error) and Header() but no WriteHeader should produce WriterOutput (it's an io.Writer) but not HTTPResponseWrite. No test fixture exercises this boundary.

2. Near-miss Write signatures

matchesWriteSignature verifies Write([]byte) (int, error) exactly. The following near-misses should all be rejected, but none are tested:

• Write([]byte) int — missing error return
• Write(string) (int, error) — wrong param type
• Write([]byte) (int64, error) — wrong int type
• Write([]byte, int) (int, error) — extra param

Suggested Fixtures

// PartialResponseWriter has Write + Header but no WriteHeader.
// Should produce WriterOutput, NOT HTTPResponseWrite.
type PartialResponseWriter struct{}
func (p *PartialResponseWriter) Header() http.Header { return nil }
func (p *PartialResponseWriter) Write(b []byte) (int, error) { return len(b), nil }

func HandlePartialRW(w *PartialResponseWriter) {
    w.Write([]byte("ok"))
}

An internal test file (p1effects_internal_test.go) could also construct synthetic types.Signature values to unit test matchesWriteSignature directly against the near-miss cases without needing full package loading.

Rationale

These are defense-in-depth tests that lock down the contracts established in #109 and #132. The current implementation is correct, but without explicit edge case coverage a future refactor could silently break the boundary conditions.

[sonupreetam]

Partial coverage landed in #193:

PR #193 adds integration-level tests for two of the cases listed here:

• Partial method set — PartialResponseWriter (Write + Header, no WriteHeader): asserts WriterOutput is emitted, HTTPResponseWrite is not.
• Near-miss Write signature — WrongWriteReturn (Write([]byte) int): asserts neither WriterOutput nor HTTPResponseWrite is emitted, with an explicit len(effects) == 0 guard against vacuously true checks.

Still open (not covered by #193):

1. Write(string) (int, error) — wrong param type
2. Write([]byte) (int64, error) — wrong int return type
3. Write([]byte, int) (int, error) — extra param
4. Internal unit test for matchesWriteSignature directly (p1effects_internal_test.go)

Keeping this issue open for the remaining items.