### What problem does this PR solve?
Implement:
1. `/api/v1/datasets/<dataset_id>/documents/<document_id>/chunks GET`
2.
`/api/v1/datasets/<dataset_id>/documents/<document_id>/chunks/<chunk_id>
PATCH`
3. `/api/v1/datasets/<dataset_id>/documents/<document_id>/chunks PATCH`
### Type of change
- [x] New Feature (non-breaking change which adds functionality)
## Summary
Stabilizes the Go unit-test surface so the test suite can run reliably
in CI and locally via \`bash build.sh --test\`.
## Verification
\`\`\`bash
bash build.sh --test -- -count=10 -run TestWithCancel_SequentialAgent
./internal/harness/core/
bash build.sh --test -- -count=5 -run TestSiliconflowChatExtracts
./internal/entity/models/
bash build.sh --test # full suite
\`\`\`
All previously failing packages (\`admin\`, \`cli\`, \`handler\`,
\`parser\`,
\`router\`, \`service\`, \`service/chunk\`) now build and test
successfully.
\`TestWithCancel_SequentialAgent\` passes 10/10 (was flaky). SiliconFlow
reasoning test passes after switching the assertion to the SiliconFlow
wire
format.
---------
Co-authored-by: Claude <noreply@anthropic.com>
Implements POST /api/v1/searchbots/ask in Go with streaming SSE,
citations, and think-tag processing. 23 files, 90+ unit tests.
---------
Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>
