feat(rds-data): typed-result fidelity for rich column types (batch 3)

[vieiralucas]

Batch 3 of the RDS Data API (rds-data). Closes the typed-result gap for non-scalar column types.

Problem

ExecuteStatement decoded BOOL/INT/FLOAT/BYTEA/text correctly, but every other Postgres column (NUMERIC, TIMESTAMP, TIMESTAMPTZ, DATE, TIME, UUID, JSON/JSONB) fell through to the String decode arm. Those columns use a binary wire format that does not decode as String, so try_get failed and the field was silently reported as isNull — a real value disappearing from the result set.

Fix

• Postgres: decode NUMERIC (rust_decimal::Decimal), TIMESTAMP/TIMESTAMPTZ/DATE/TIME (chrono), UUID (uuid), JSON/JSONB (serde_json::Value) and render each as AWS stringValue. Timestamps use Postgres's own canonical text (fractional seconds only when present, trailing zeros trimmed).
• MySQL: format Date/Time protocol values as canonical SQL text instead of the Rust debug form.
• tokio-postgres gains with-chrono-0_4 / with-uuid-1 / with-serde_json-1; adds rust_decimal (db-tokio-postgres).

Tests

New Docker-gated e2e (rds_data_api_decodes_rich_column_types): inserts NUMERIC/TIMESTAMP/UUID/JSONB and asserts each comes back as the expected stringValue against a real Postgres container. Passes locally; the existing typed-param and transaction tests still pass.

Builds clean; clippy --all-targets -D warnings clean; fmt clean.

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Summary by cubic

Fixes typed-result gaps in rds-data ExecuteStatement by correctly decoding Postgres NUMERIC/TIMESTAMP/TIMESTAMPTZ/DATE/TIME/UUID/JSON(B) to AWS stringValue, and by rendering MySQL DATE/DATETIME/TIME as canonical SQL text. Prevents real values from collapsing to isNull and improves result fidelity.

• Bug Fixes

  • Postgres: decode via rust_decimal, chrono, uuid, and serde_json; timestamps use Postgres-style text with trimmed fractional seconds and timezone for TIMESTAMPTZ.
  • MySQL: output canonical SQL text for DATE/DATETIME/TIME (includes sign and microseconds) instead of Rust debug forms.
  • Removes the fallback String decode path that caused binary columns to return isNull.

• Dependencies

  • Enable tokio-postgres features: with-chrono-0_4, with-uuid-1, with-serde_json-1.
  • Add rust_decimal (feature db-tokio-postgres), plus chrono and uuid.

^{Written for commit 174f676. Summary will update on new commits.}