fix(rpc): make ChainGetTipset v2 work with lotus-gateway

[hanabi1224]

Summary of changes

Changes introduced in this pull request:

Reference issue to close (if applicable)

Closes #6552

Other information and links

Change checklist

• I have performed a self-review of my own code,
• I have made corresponding changes to the documentation. All new code adheres to the team's documentation standards,
• I have added tests that prove my fix is effective or that my feature works (if possible),
• I have made sure the CHANGELOG is up-to-date. All user-facing changes should be reflected in this document.

Outside contributions

• I have read and agree to the CONTRIBUTING document.
• I have read and agree to the AI Policy document. I understand that failure to comply with the guidelines will lead to rejection of the pull request.

Summary by CodeRabbit

• New Features

  • Filecoin.ChainGetTipset RPC now works with lotus-gateway.

• Improvements

  • More robust tipset selection handling with improved defaulting and serialization behavior.
  • Normalized RPC error messages for clearer, consistent error text when calling APIs.

• Documentation

  • Added changelog entry noting the lotus-gateway compatibility fix.

• Tests

  • Expanded test coverage to validate URL parsing and error-message normalization.

[coderabbitai]

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Walkthrough

Adds serde defaults for TipsetSelector fields, normalizes lotus-gateway-wrapped RPC error messages in API comparison tests, updates test endpoints to WebSocket, and adds related tests and changelog entry.

Changes

Cohort / File(s)	Summary
Serde serialization defaults src/rpc/types/tipset_selector.rs	Added default to TipsetHeight.anchor, TipsetAnchor::key and TipsetAnchor::tag serde attributes and skip_serializing_if for key, adjusting (de)serialization to accept missing/None values.
API test infra: endpoints scripts/tests/api_compare/docker-compose.yml	Switched online API comparison endpoints to use WebSocket (/ws) and added clarifying comments for online/offline blocks.
API test infra: auth checks src/rpc/mod.rs	In tests, added an HTTP AuthVerify call using the HTTP client alongside the existing WebSocket AuthVerify check.
RPC error normalization src/tool/subcommands/api_cmd/api_compare_tests.rs	Added normalized_error_message(s: &str) -> Cow<'_, str> to strip lotus-gateway RPC prefixes from error messages and updated test logic to use normalized messages; includes unit tests.
Tests & changelog src/utils/mod.rs, CHANGELOG.md	Added a WebSocket URL parsing test case and improved assertion messages; added changelog entry noting Filecoin.ChainGetTipset lotus-gateway compatibility fix.

Estimated code review effort

🎯 3 (Moderate) | ⏱️ ~25 minutes

Possibly related PRs

• chore: fix GetActorEventsRaw tests #6319: RPC error handling changes in api_compare_tests.rs to accommodate lotus-gateway wrapped messages.
• feat(rpc): V2 RPC methods StateGetActor and StateGetID #6312: Earlier changes to src/rpc/types/tipset_selector.rs serde behavior for tipset selector types.
• feat(api): implement the ChainGetFinalizedTipset API #6133: Modifications around API comparison tests affecting similar test areas and comparisons.

Suggested reviewers

• LesnyRumcajs
• sudo-shashank

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 70.00% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title accurately describes the main change: fixing RPC compatibility for ChainGetTipset v2 with lotus-gateway, which aligns with the primary objectives.
Linked Issues check	✅ Passed	All code changes address the core issue #6552 requirements: serde attributes added for proper deserialization, WebSocket support verified, error normalization for gateway compatibility, and HTTP client testing added.
Out of Scope Changes check	✅ Passed	All changes are directly related to fixing ChainGetTipset v2 compatibility with lotus-gateway; no out-of-scope modifications detected.

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✨ Finishing touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment
• Commit unit tests in branch hm/fix-chain-get-tipset-v2-gateway-compatibility

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[coderabbitai]

Actionable comments posted: 1

🤖 Fix all issues with AI agents

In `@CHANGELOG.md`:
- Line 66: Update the changelog entry that currently references issue `#6553` so
it points to issue `#6552` instead: replace the link and number in the line "-
[`#6553`](https://github.com/ChainSafe/forest/issues/6553):
`Filecoin.ChainGetTipset` now works with `lotus-gateway`." to use "#6552" and
the URL "https://github.com/ChainSafe/forest/issues/6552" so the entry reads "-
[`#6552`](https://github.com/ChainSafe/forest/issues/6552):
`Filecoin.ChainGetTipset` now works with `lotus-gateway`."

[LesnyRumcajs]

no green checkmark, no review