fix(openai): decode Responses top_p with arbitrary-precision JSON

[mzdora]

Description

Fixes #2493.

When a dependency enables serde_json/arbitrary_precision, Serde buffers a fractional top_p as a map through the flattened Responses metadata and the untagged streaming event. Decoding it directly as f64 rejects otherwise valid responses, including the terminal frame carrying token usage.

Deserialize top_p through serde_json::Number before converting it to f64. The public field and serialized request shape stay unchanged; missing/null values remain supported, and invalid types or numbers outside the f64 range still fail.

Changelog

• (openai) Fix Responses decoding and token-usage preservation when serde_json/arbitrary_precision is enabled.

Migration

None.

Type of change

• Bug fix

Testing

• Before the fix: the non-streaming response and completed streaming-event regressions both failed with arbitrary precision enabled; the two compatibility tests passed.
• cargo test --locked -p rig-core --lib providers::openai::responses_api::tests:: — 92 passed.
• cargo test --locked -p rig-core --lib --features serde_json/arbitrary_precision providers::openai::responses_api::tests:: — 92 passed.
• cargo clippy --locked -p rig-core --lib --tests --features serde_json/arbitrary_precision -- -D warnings — passed.
• cargo fmt --all -- --check and git diff --check — passed.
• CI now explicitly runs the four regressions with arbitrary precision enabled.

No cassettes were recorded or changed. These tests reproduce a Serde feature-unification failure using raw JSON, without requiring a provider request. Independent full-diff review found no actionable issues. Comprehensive CI has not run on this branch yet.

Checklist

• My code follows the style guidelines of this project.
• I have added tests that prove the fix and preserve error handling.
• I have completed minimal relevant local checks and reported them separately from CI.
• I did not edit generated changelog or migration files.

[mzdora]

@gold-silver-copper @joshua-mo-143 hi, could you please review?

[gold-silver-copper]

Superseded by #2531