feat(rds-data): conformance coverage + docs/metadata sync (batch 4)

[vieiralucas]

Final batch of the RDS Data API (rds-data) service: bring it into the Level-1 conformance harness and sync every metadata surface.

Conformance

• aws-models/service-map.json: register rds-data so the probe loads its Smithy model.
• probe/service_detection.rs: map rds-data to the restJson1 REST protocol. The @http-trait request builder then routes ExecuteStatement -> POST /Execute, etc. (it previously fell through to the awsQuery default and every request 404'd).
• conformance-baseline.json: add rds-data at 253/253 variants, totals bumped to 124,508.

The 253 variants are environment-independent: they exercise the no-DB error path, which returns the modeled BadRequestException that the probe counts as conformant (declared in the op's Smithy error_shapes). So they reproduce identically in CI without Docker — no gaming, the real responses are shape-correct. Verified: cargo run -p fakecloud-conformance -- check passes (no regressions), -- audit passes, and the service_protocol_covers_every_shipped_service test passes.

Docs / metadata

• parity.md: new RDS Data API row (6 ops) + header bump to 42 services / 3,723 operations / 124,508 variants.
• Swept every check-doc-counts.sh-tracked surface (README, AGENTS, llms.txt/llms-full.txt, vs/*, service indexes, FAQ, glossary, ...) to the new totals.
• Added an RDS Data API row to the README service table + LocalStack-comparison table, the docs/services/_index.md table, and supported-services.md.

check-doc-counts.sh passes. No source-behavior change — this is harness registration + documentation.

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Summary by cubic

Adds RDS Data API (rds-data) to the conformance harness and fixes protocol detection so its REST endpoints route correctly. Totals now: 42 services and 124,508 passing variants; no behavior changes.

• New Features
  • Register rds-data in aws-models/service-map.json and map it to restJson1 in the probe (e.g., ExecuteStatement -> POST /Execute).
  • Add rds-data to conformance-baseline.json at 253/253 variants.
  • Update docs and service tables (README, parity, supported-services, vs/*) with RDS Data API (6 ops) and refreshed totals.

^{Written for commit 9a7372a. Summary will update on new commits.}