Preserve Prebid bidder parameter string types

[ChristianPavilonis]

Summary

• preserve nested JSON scalar types in Prebid bidder parameter override containers
• decode JSON environment overrides once without recursively coercing numeric-looking strings
• add regression coverage for TOML-to-runtime round trips and JSON environment overrides

Closes #932

Testing

• cargo fmt --all -- --check
• cargo test --package trusted-server-core --target aarch64-apple-darwin bid_param_overrides
• cargo test-axum
• cargo test-fastly