[codex] Harden bamltype parity tests and decouple from legacy bridge oracle

[darinkishore]

Summary

This PR hardens bamltype parity testing so it no longer depends on runtime comparisons against the legacy baml-bridge path.

Key changes:

• Replaced the legacy bridge oracle test with frozen fixture contract coverage in crates/bamltype/tests/contract_frozen_oracle.rs.
• Converted UI message parity checks to fixture digest assertions in crates/bamltype/tests/contract_bridge_ui_messages.rs.
• Added fixture-backed parity suites for golden schemas, render options, and property behavior:
  • crates/bamltype/tests/golden_parity.rs
  • crates/bamltype/tests/render_options_parity.rs
  • crates/bamltype/tests/property_parity.rs
• Removed baml-bridge as a bamltype dev-dependency.
• Moved test-suite guidance to crate-local docs at crates/bamltype/AGENTS.md and removed stale testing TODO at crates/baml-bridge/TODO.md.

Legacy usage status

• baml-bridge and baml-bridge-derive are still workspace members.
• No other crate currently depends on them in Cargo.toml.
• bamltype parity tests no longer import or call into the legacy crates.

Validation

• cargo fmt --all --check
• cargo clippy --workspace --all-targets --all-features -- -D warnings
• cargo test --workspace

All commands passed on this branch.