docs: improve agent guidance on issue references in regression tests

[opieter-aws]

Description

Tighten 'comments in tests' guidance so the rule is unambiguous: an issue reference belongs only on a regression test for a discovered bug, and a test written as part of feature development carries none. My agent would add issue guidance to tests for feature development.

The canonical rule in the root AGENTS.md is narrowed, and the per-SDK TESTING.md files are brought in line. The TypeScript TESTING.md had no "Comments in Tests" section at all — only the Python guide did — so agents working in strands-ts never saw this rule in their own testing doc; this adds the parity section.

Related Issues

N/A

Documentation PR

This change is entirely to agent-facing guidance files (AGENTS.md, docs/TESTING.md); no site/ documentation changes are needed.

Type of Change

Documentation update

Testing

• I ran hatch run prepare

Checklist

• I have read the CONTRIBUTING document
• I have reviewed and understand every line of code in this PR, including any generated by AI tools, and I can explain why it works
• My change is focused and reasonably small; I have split unrelated work into separate PRs
• I have added any necessary tests that prove my fix is effective or my feature works
• I have updated the documentation accordingly
• I have added an appropriate example to the documentation to outline the feature, or no new docs are needed
• My changes generate no new warnings
• Any dependent changes have been merged and published

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[github-actions]

Assessment: Comment

Focused, easy-to-follow documentation change. The rule is clearly narrowed — an issue reference belongs only on a regression test for a discovered bug — and the TypeScript parity section is a welcome addition. Two minor points before merge, neither blocking.

Review Categories

• Formatting: A stray blank line was added inside the tight bullet list in AGENTS.md, breaking list consistency (inline comment).
• Cross-file parity: The TS guide says "issue/PR reference" while the canonical AGENTS.md and Python guide say only "issue reference"; worth aligning given this PR's parity goal (inline comment).

Nice catch closing the gap where strands-ts had no "Comments in Tests" section at all.