test: Add seeded taint trace for db_query -> http_post exfiltration (#4)

[IronLad123]

Closes #4

Summary

Adds a new seeded OTLP-style taint trace fixture (tests/fixtures/seeded/taint_db_to_http.json) exercising a db_query source returning a secret token segment (SECRET_DB_TOKEN_998877, ≥12 chars) that reaches an http_post sink with no intervening sanitizer span.

Acceptance Criteria Covered

• Seeded Fixture Added: tests/fixtures/seeded/taint_db_to_http.json created with a db_query tool returning SECRET_DB_TOKEN_998877 and a subsequent http_post sink carrying that segment in its payload.
• Unit & Acceptance Tests:
  • Added test_seeded_db_to_http_taint_flow_fires in tests/test_taint.py asserting exactly one taint error finding with span IDs ("s-db", "s-http").
  • Added ("taint_db_to_http.json", "taint") to FAILING in tests/test_acceptance.py to cover full acceptance suite.
• Verification: All 49 tests pass (pytest green); ruff check clean.

[bamdadd]

Thank you @IronLad123! This seeded db_query -> http_post fixture is a great addition to the taint corpus - a realistic secret-exfiltration flow (SECRET_DB_TOKEN reaching an HTTP POST with no sanitizer between). Wired into both the unit and acceptance suites. Squash-merged to main as 15273b1.

[bamdadd]

Merged via 15273b1. Thanks!