Fix: return HTTP 409 for duplicate datasets and jobs

[shollands-sc]

Summary

When creating a dataset or job that already exists, the emulator returns HTTP 500 (InternalServerError) instead of the correct HTTP 409 (Conflict). This causes two problems for BigQuery client libraries:

1. Retry storm: The Python client treats 500 as a transient error and retries with exponential backoff for up to 600 seconds (~10 minutes per affected call)
2. exists_ok=True bypass: The Python client's exists_ok parameter only suppresses 409 errors, so the 500 bypasses it entirely

Approach

This follows the existing ErrDuplicatedTable pattern already in the codebase (internal/metadata/dataset.go + server/handler.go:2609):

• Add ErrDuplicatedDataset and ErrDuplicatedJob sentinel errors in internal/metadata/project.go
• Wrap with %w in AddDataset / AddJob for errors.Is support
• Check errors.Is in ServeHTTP and use errDuplicate() for the HTTP response

Addressing feedback from #184

Per @goccy's review comments on #184:

• "Please use errDuplicate function in error.go" — Done. We use the existing errDuplicate() in the handlers, consistent with how ErrDuplicatedTable is already handled
• "It does not seem necessary to change to *ServerError type" — Done. Handle() method signatures are unchanged; the errors.Is check happens in ServeHTTP

Changes

File	Change
internal/metadata/project.go	Add ErrDuplicatedDataset, ErrDuplicatedJob sentinels; wrap with %w
server/handler.go	datasetsInsertHandler.ServeHTTP and jobsInsertHandler.ServeHTTP check errors.Is → errDuplicate()
server/server_test.go	Add TestDuplicateDataset (mirrors existing TestDuplicateTable)

Fixes #256
Supersedes #184

Test plan

• TestDuplicateDataset — verifies HTTP 409 on duplicate dataset creation
• TestDuplicateTable — existing test still passes (no regression)
• TestDataset — existing test still passes (no regression)
• go build ./... — compiles cleanly