fix: preserve declared response headers in operation metadata

[sansynx]

Summary

Preserve declared response headers in the OpenAPI resolver, including referenced responses, referenced headers, and header schema references

While investigating cloudflare/forge#20, I found that the Stream TUS operation correctly declares its upload URL in the Location response header. extractResponses currently drops the entire headers map, so downstream generators cannot discover it

This adds optional header metadata without changing the shape of responses that have no declared headers. The CLI integration is separate, this change alone does not fix the command

Validation

• Added a regression test and observed it fail before the resolver change
• Forge package tests: 38 passed after rebasing onto current main
• Forge package typecheck passed
• Targeted Oxfmt check and package Oxlint passed
• Independent read-only code review found no actionable issues

AI assistance was used for implementation and validation

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