test: add unit tests for responses writer, templ, logger

[SVilgelm]

Add comprehensive tests for the HTTP response writer, template
execution, and structured logging middleware.

Key changes:

• Add TestResponsesWriter_FileAndJSONHeader to verify serving file
  contents, status code, JSON content-type, and custom headers.
• Add TestResponsesWriter_RepeatLogic to assert Repeat behavior keeps
  a response for N calls before advancing to the next response.
• Add TestResponsesWriter_NotFoundWhenExhausted to ensure a 404 is
  returned when no responses remain.
• Add TestResponsesWriter_FileReadError to check 500 on missing file
  and that the error message includes the filename.
• Add TestResponsesWriter_JSONDoesNotOverrideExistingContentType to
  confirm IsJSON does not overwrite an explicitly set Content-Type.
• Add TestExecuteTemplate_Success and TestExecuteTemplate_ParseError to
  validate template execution and parse error handling.
• Add TestStructuredLogger_BasicFields and helpers to validate JSON
  structured logging output and basic middleware behavior.

Also add testLogger helper for building a JSON logger writing to a
buffer. Tests use httptest, temporary files, and explicit header
assertions to make behavior deterministic.

[Copilot]

Pull Request Overview

This PR adds comprehensive unit tests for HTTP response handling, template execution, and structured logging middleware. The tests focus on verifying core functionality including file serving, response repetition logic, error handling, and JSON logging output.

• Add thorough test coverage for ResponsesWriter behavior including file serving, repeat logic, and error scenarios
• Add template execution tests for both success and error cases
• Add structured logging middleware tests with JSON output validation

---

_{Tip: Customize your code reviews with copilot-instructions.md. Create the file or learn how to get started.}