Return 413 Content Too Large for oversized response bodies

[Copilot]

When response bodies exceed MaxBodyBytes, the middleware was either returning 500 (enforce mode) or committing truncated, potentially corrupt data (audit mode).

Changes

• Return HTTP 413 consistently when bw.tooLarge is detected, regardless of validation mode
• Remove mode-dependent branching that committed partial responses
• Use http.StatusRequestEntityTooLarge with descriptive error payload

// Before: mode-dependent handling
if cfg.Mode == ResponseValidationEnforce {
    c.Status(http.StatusInternalServerError) // 500
    return
}
_ = bw.commit() // commits truncated data in audit mode

// After: consistent 413 response
c.Status(http.StatusRequestEntityTooLarge) // 413
c.Writer.Write([]byte(fmt.Sprintf(`{"error":"response body too large","detail":%q}`, msg)))

Logging preserved for observability.

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