feat(protocol): add bounded Codex Responses parser

[tiammomo]

Closes #40

Add a route-free, fail-closed Codex Responses request parser and content-free fixture pinned to codex-cli 0.148.0-alpha.21.

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Covers all 15 official top-level request-builder fields, observed optional omissions and item IDs, bounded string client metadata, and strict rejection of unsupported multimodal, hosted-tool, structured-output, and unknown shapes. Declares openai-responses and /v1/responses identity but intentionally does not register a route or normalize into Exchange; those remain in #39/#38.

Verified with scripts/check-all.sh.

Areas for careful review:

• No known behavior changes outside the reported bug.

Implementation assistance: an external coding workspace was used to prepare the change and its
tests. tiammomo reviewed the final diff, understands it, and takes responsibility
for this contribution.