fix: retry generic transient upstream errors

[grikomsn]

Summary

• retry the gateway's explicit generic 500 Internal server error response with the existing bounded backoff
• retain non-transient 500 failures as terminal
• add regression coverage and a patch changeset

Issue status

• Addresses [Bug]: MiMo V2.5 returns upstream 500 Internal server error #13: the reported generic 500 now gets two bounded retries before it is surfaced.
• Related [Bug]: DeepSeek V4 Flash rejects tool-result history with ChatMessageRole.TOOL validation error #12: current main's tool-history serialization was verified in a real VS Code Extension Development Host with DeepSeek V4 Flash; the tool round-trip completed without the reported 422.

Verification

• npm test (71 passing)
• npm run package
• VS Code Extension Development Host: provider registered, native Thinking Effort control present, and a DeepSeek V4 Flash tool-result round-trip completed successfully.

[pullfrog]

Important

The generic 500 retry path still does not match the error reported by the issue, so this PR should not merge until the call-site contract is covered.

Reviewed changes This review covers the retry classification update and its release and regression-test coverage.

• Generic transient 500 classification Extends the retry predicate to recognize the gateway's exact Internal server error text for HTTP 500 responses.
• Release and regression coverage Adds a patch changeset and a unit assertion for the new classification.

  ^{｜ Fix all ➔ ｜ Fix 👍s ➔ ｜ View workflow run ｜ Using GPT Luna ｜ 𝕏}

[pullfrog]

✅ No new issues found.

Reviewed changes This incremental review covers the follow-up that aligns generic 500 classification with the actual formatted provider error.

• Matched formatted upstream errors Updated the transient 500 predicate to recognize the responseError message shape while retaining terminal handling for unrelated 500 details.
• Added regression coverage Pinned the exact issue-shaped error message so the prior call-site mismatch would fail the test.

  ^{｜ View workflow run ｜ Using GPT Luna ｜ 𝕏}