[Test Coverage] test: improve config-file.ts coverage from 55% to 99% (#2108)

* test: improve config-file.ts coverage from 55% to 99%

Add comprehensive tests for all uncovered code paths in config-file.ts:

- validateAwfFileConfig: non-object root, all invalid field types across
  every config section (network, apiProxy, security, container,
  environment, logging, rateLimiting), unknown key rejection, and
  valid combinations
- loadAwfFileConfig: YAML file loading (.yml extension), stdin YAML
  fallback, extensionless file JSON/YAML fallback, parse error messages,
  validation error messages with path/stdin labels
- mapAwfFileConfigToCliOptions: all fields (security, container, env,
  logging, rateLimiting), empty array handling, undefined passthrough
- applyConfigOptionsInPlaceWithCliPrecedence: all-apply, undefined-skip,
  and overwrite-without-CLI-flag scenarios

Coverage improvement:
- config-file.ts: 55.44% → 99.48% statements, 47.75% → 99.59% branches
- Overall: 83.45% → 85.65% statements, 74.72% → 79.91% branches

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Update src/config-file.test.ts

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

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Co-authored-by: github-actions[bot] <41898282+github-actions[bot]@users.noreply.github.com>
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Co-authored-by: Landon Cox <landon.cox@microsoft.com>
Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>
Co-authored-by: lpcox <15877973+lpcox@users.noreply.github.com>

Author: 4 people