fix(spider-storage): Reserve UNAVAILABLE gRPC status code for network connection errors.

[LinZhihao-723]

Description

UNAVAILABLE was being returned by user code in two storage handlers, which made it ambiguous: a genuine transport failure (lost or unestablished connection) and an application-level error both surfaced as UNAVAILABLE, so the execution-manager client could not tell them apart. Per the gRPC status-code guidance, only a small set of codes are appropriate for user code to return, and UNAVAILABLE is best left to the transport. This PR remaps the two offending cases onto application-appropriate codes and updates the execution-manager client to treat UNAVAILABLE as a transport error. It closes the follow-up flagged at the end of #354.

Server changes (spider-storage/src/grpc.rs)

• job_orchestration_service_error_handler: a fatal cache-internal error now returns INTERNAL instead of UNAVAILABLE (it still fires the cancellation token and restarts the service).
• task_instance_management_service_error_handler: a request from a stale session now returns NOT_FOUND instead of UNAVAILABLE. The other mappings are unchanged.
• Updated the handler doc comments to match.

Client changes (spider-execution-manager/src/client/grpc/storage.rs)

• status_to_error now maps Code::Unavailable to StorageResponseError::Transport, so a lost or unestablished connection is reported as a transport failure rather than as an application error.
• Stale-session detection is remapped to follow the server: Code::NotFound → StorageResponseError::StaleSession. This is required for correctness — without it, the server's new NOT_FOUND stale-session response would fall through to the generic Server case and stale-session handling would silently break.
• Updated the unit tests: replaced status_maps_unavailable_to_stale_session with status_maps_not_found_to_stale_session, and added status_maps_unavailable_to_transport.

Checklist

• The PR satisfies the contribution guidelines.
• This is a breaking change and that has been indicated in the PR title, OR this isn't a
  breaking change.
• Necessary docs have been updated, OR no docs need to be updated.

Validation performed

• Ensure all workflows pass.

Summary by CodeRabbit

• Bug Fixes
  • Improved gRPC error responses for storage-related issues, so some failures now return more appropriate status codes.
  • Session-related errors are now reported more consistently, helping clients distinguish missing, stale, and transport-related problems.
  • Updated error messages to better reflect the actual issue returned to callers.

[LinZhihao-723]

@sitaowang1998 Please check this PR since it may affect the error handling in the coming service implementations.

[coderabbitai]

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Repository UI

Review profile: CHILL

Plan: Pro

Run ID: 8b09e2dd-f878-456d-b585-46fe25175367

📥 Commits

Reviewing files that changed from the base of the PR and between f314e98 and ef3f167.

📒 Files selected for processing (1)

• components/spider-execution-manager/src/client/grpc/storage.rs

Walkthrough

gRPC error-code mappings were updated in the storage service and the execution-manager client. The storage service now returns INTERNAL and NOT_FOUND for specific error paths, and the client now maps NOT_FOUND to StaleSession and UNAVAILABLE to Transport.

Changes

gRPC status mapping updates

Layer / File(s)	Summary
Storage service handlers components/spider-storage/src/grpc.rs	job_orchestration_service_error_handler now returns INTERNAL for cache-internal failures, and task_instance_management_service_error_handler now returns NOT_FOUND for stale sessions; the code comments are updated to match.
Execution-manager status mapping and tests components/spider-execution-manager/src/client/grpc/storage.rs	status_to_error now maps NOT_FOUND to StaleSession and UNAVAILABLE to Transport, and the tests cover the updated mappings and invalid-argument message handling.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~10 minutes

Possibly related PRs

• y-scope/spider#340: Shares the execution-manager gRPC storage client error-code/status mapping changes in components/spider-execution-manager/src/client/grpc/storage.rs.
• y-scope/spider#354: Also changes status_to_error in the same client file, including the UNAVAILABLE/NOT_FOUND translation logic and tests.

Suggested reviewers

• sitaowang1998

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Docstring Coverage	✅ Passed	Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Title check	✅ Passed	The title accurately reflects the main change: reserving UNAVAILABLE for transport-level connection errors.
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.

✨ Finishing Touches

🧪 Generate unit tests (beta)

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