restore: stop bulk insert polling on context cancellation

[huanghaoyuanhhy]

Fixes #1157

Summary

The bulk insert state polling loops in checkBulkInsertViaGrpc and checkBulkInsertViaRestful (core/restore/coll_task.go) used for range time.Tick(...) and never checked ctx.Done() in the loop itself. Cancellation only surfaced indirectly, when the next GetBulkInsertState RPC failed with a context error, so a cancelled restore task kept polling for up to another poll interval (and, in a stale-success race at the moment of cancellation, longer). The loops also leaked their tickers.

Changes

• Both loops now use time.NewTicker with select { <-ctx.Done(); <-ticker.C } between polls, mirroring the secondary restore implementation in core/restore/secondary/coll_dml_task.go
• Drop the trailing return errors.New(...) lines, which became unreachable (govet) once the loops were rewritten as unbounded for {}
• Polling cadence and no-progress watchdog behavior are unchanged

Test plan

• go test ./core/restore/ passes
• golangci-lint clean on the changed package

[sre-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: huanghaoyuanhhy

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[codecov-commenter]

Codecov Report

❌ Patch coverage is 0% with 14 lines in your changes missing coverage. Please review.
✅ Project coverage is 42.71%. Comparing base (2309945) to head (aead0c3).
⚠️ Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
core/restore/coll_task.go	0.00%	14 Missing ⚠️

❌ Your patch status has failed because the patch coverage (0.00%) is below the target coverage (80.00%). You can increase the patch coverage or adjust the target coverage.
❌ Your project status has failed because the head coverage (42.71%) is below the target coverage (80.00%). You can increase the head coverage or adjust the target coverage.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1158      +/-   ##
==========================================
+ Coverage   42.50%   42.71%   +0.20%     
==========================================
  Files         136      134       -2     
  Lines       12181    12229      +48     
==========================================
+ Hits         5178     5224      +46     
+ Misses       6624     6611      -13     
- Partials      379      394      +15     

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