docs: add conformance fixture contribution guidance

[kmishra1204]

Summary

• add a dedicated section in CONTRIBUTING for conformance fixture proposals
• define minimum metadata to improve cross-SDK interoperability validation

Context

Closes #1957.

Notes

Docs-only change.

[gemini-code-assist]

Code Review

This pull request updates CONTRIBUTING.md to add a new section outlining recommended patterns and minimum metadata for conformance fixture contributions. The feedback suggests capitalizing the first letter of each bullet point in the new section to maintain consistency with other lists in the repository.

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[gemini-code-assist]

For consistency with other lists in the repository (such as those in README.md), the bullet points should start with a capital letter.

Suggested change

	- protocol area covered (for example: discovery, lifecycle, errors, streaming)
	- expected request/response shape or event sequence
	- pass/fail criteria that can be checked across SDKs
	- edge case rationale (why this fixture matters for interoperability)
	- Protocol area covered (for example: discovery, lifecycle, errors, streaming)
	- Expected request/response shape or event sequence
	- Pass/fail criteria that can be checked across SDKs
	- Edge case rationale (why this fixture matters for interoperability)

[kmishra1204]

Thanks for the pipeline checks. I just updated this PR metadata to satisfy repo gates:

• title normalized to conventional commit style (docs:)
• PR description updated with an explicit linked issue (Closes #1957)

Validate PR Title and Check linked issues are now passing.

I still see Check Spelling failing due the workflow action security advisory (not due changed docs text). Happy to re-run once the workflow is updated, or apply any maintainer-preferred workaround.

[kmishra1204]

Applied the review wording fix in faec896.

What changed:

• Capitalized the first word in each new conformance fixture metadata bullet for consistency with the surrounding contribution guide style.

Local validation:

• npx --yes markdownlint-cli --config .github/linters/.markdownlint.json CONTRIBUTING.md
• git diff --check