fix(bitbucket): tolerate null datacenter comment replies

[KatalKavya96]

Summary

• Hardens Bitbucket Data Center threaded comment normalization when the API returns an explicit comments: null value.
• Treats both missing and null nested comment.comments fields as no replies by using raw.get("comments") or [].
• Adds a regression test for the comments: null case so the discussion normalizer does not crash on this payload shape.

Type of change

• Skill change (.claude/skills/<name>/) — eval fixtures updated below
• Tool / bridge contract (tools/<system>/*.md)
• Python package (tools/*/ with pyproject.toml)
• Groovy reference impl
• Cross-cutting (RFC, AGENTS.md, sandbox, privacy-LLM)
• Documentation (docs/, README.md, CONTRIBUTING.md)
• Project template (projects/_template/)
• CI / dev loop (prek, workflows, validators)
• Other:

Test plan

• prek run --all-files passes
• For Python packages touched: uv run pytest / ruff check / mypy passes
• For Groovy bridges touched: command-line invocation tested end-to-end
• For skill changes: eval suite passes for the affected skill
  (PYTHONPATH=tools/skill-evals/src python3 -m skill_evals.runner tools/skill-evals/evals/<skill>/)
• For skill behaviour changes: a new or updated eval fixture is included in this PR
  (a regression test for the bug fixed / the behaviour added — see CONTRIBUTING.md)
• Other: verified targeted Bitbucket tests with PYTHONPATH=src uv run --group dev pytest tests/test_bitbucket.py

RFC-AI-0004 compliance

• HITL — any new mutation is gated on explicit user confirmation
• Sandbox — no new unrestricted host access; network reach declared in the adapter
• Vendor neutrality — keeps the Bitbucket bridge within the existing vendor-neutral contract:change-request / partial read-only path without adding new Bitbucket-specific capability prose.
• Conversational + correctable — agentic-override path documented if behaviour is adopter-tunable
• Write-access discipline — this is a read-only normalization hardening change and does not add any outbound write or mutation path.
• Privacy LLM — private content does not reach a non-approved LLM; redactor invoked where needed

Linked issues

Refs #606

Notes for reviewers

This follows up on the non-blocking robustness note from the merged Bitbucket discussion PR.

Bitbucket Data Center nested replies are already flattened through _datacenter_comment_tree; this PR only hardens that path for payloads where the API returns "comments": null instead of omitting the field or returning an empty list.

The behaviour is unchanged for normal threaded replies.

[KatalKavya96]

Hi @potiuk, this is a quick follow-up fix for the non-blocking robustness suggestion left after the previous Bitbucket discussion PR was merged.

It only hardens _datacenter_comment_tree for the comments: null case and adds a regression test for it. If this looks good, I’d appreciate a quick review/merge so I can move on to the next Bitbucket implementation.

Thanks again.