test(ingestion): add fixture-based end-to-end coverage for fetch runtime and persistence

[CrisSTEM]

Summary

This PR adds fixture-based end-to-end ingestion coverage for the v0.3.0 runtime and persistence path.

The repository already had source-specific integration coverage for GitHub, news RSS, changelog, runtime dispatch, and CLI non-zero behavior. This PR adds the missing cross-source runtime-level tests that exercise real fetchers against mocked upstream responses and verify persisted rows in the same temporary SQLite database.

Linked issue(s)

• Closes test(ingestion): add fixture-based end-to-end coverage for fetch runtime and persistence #33

Milestone fit

This PR strengthens the v0.3.0 ingestion milestone by adding deterministic end-to-end coverage for the live runtime path without introducing any new product behavior.

It stays inside milestone scope by:

• using temp SQLite databases
• using fixture/mocked upstream responses only
• validating persisted fetch_runs, metric_snapshots, and content_items
• proving rerun dedupe for content items
• proving mixed success/failure recording at runtime level

What changed

• added internal/fetch/runtime_e2e_test.go
• added a cross-source success-path runtime test that:
  • runs GitHub ingestion
  • runs news RSS ingestion twice
  • runs changelog RSS ingestion twice
  • verifies persisted rows across fetch_runs, metric_snapshots, and content_items
  • verifies rerun dedupe preserves first-write content rows
• added a mixed success/failure runtime test that:
  • runs multiple GitHub bindings with one simulated upstream failure
  • verifies successful bindings still persist rows
  • verifies failed bindings record failed fetch_runs
  • verifies the failing binding does not persist snapshots
  • verifies subsequent news and changelog runs still succeed in the same database
• reused the existing deterministic httptest helpers and fixture pack patterns already present in internal/fetch

Why this shape

Most of the issue scope was already covered in main through:

• source-specific fetcher integration tests
• runtime dispatch tests
• CLI fetch exit-code tests

Given that local repository state, this PR keeps the change minimal and auditable by adding the missing multi-source E2E runtime coverage instead of refactoring the existing test suite.

Out of scope

• live network ingestion tests
• scheduler coverage
• frontend or API tests
• product/runtime behavior changes
• release documentation beyond test coverage

Validation

• go test ./internal/fetch -count=1
• go test ./... -count=1
• go vet ./...
• python3 ./scripts/trim-whitespace.py --check
• make ci

Documentation impact

• No docs update needed

Reviewer focus

• internal/fetch/runtime_e2e_test.go
  • cross-source runtime coverage
  • same-database persistence assertions
  • rerun dedupe assertions
  • mixed success/failure recording
• confirm the new tests complement, rather than duplicate, existing source-level integration coverage

Auditability checklist

• Branch name follows project convention
• PR scope matches the linked issue and milestone
• No live-network CI dependency was introduced
• Relevant tests were added for runtime-level persistence coverage
• Deferred work is explicitly called out
• No unimplemented capabilities are implied

Notes for merge

Preferred merge method:

• Issue PRs: Rebase and merge
• Release aggregation PRs: Merge commit
• Use Squash and merge only when the branch became noisy and preserving intermediate commits no longer adds value