feat(providers): support OpenRouter request parameters

[sunilkumarvalmiki]

Resolves #8646.

Summary

• Add optional OPENROUTER_PARAMETERS provider config.
• Accept both YAML object values and JSON string values for advanced OpenRouter request fields.
• Merge custom OpenRouter parameters without overwriting Goose-managed request fields.
• Document YAML and JSON string examples for verbosity, reasoning, and plugins.

Testing

• cargo fmt --all --check
• cargo check -p goose --no-default-features --features rustls-tls,local-inference --lib
• CARGO_BUILD_JOBS=1 cargo test -p goose --no-default-features --features rustls-tls,local-inference --lib openrouter_parameters

[DOsinga]

Eh, actually can you rework this to not duplicate the existing request_params infrastructure? ModelConfig.request_params already gets merged into the OpenAI-compatible payload by create_request() in formats/openai.rs — so the simpler approach is to parse OPENROUTER_PARAMETERS in from_env() and feed it into model_config.request_params there. That way you don't need the apply_openrouter_parameters function, the extra field on the provider struct, or the second merge path with subtly different collision semantics (or_insert vs the existing insert-with-reserved-key-check). Most of the tests can go too since they're just testing serde_json basics.

[sunilkumarvalmiki]

Reworked this to use the existing ModelConfig.request_params flow in OpenRouterProvider::from_env() instead of a provider-specific payload merge path. The provider field and apply_openrouter_parameters helper are removed, and the tests now cover parsing plus propagation into both the primary and fast model configs.

Local checks run:

• cargo fmt --all --check
• CARGO_BUILD_JOBS=1 cargo test -p goose --no-default-features --features rustls-tls,local-inference --lib openrouter_parameters
• CARGO_BUILD_JOBS=1 cargo check -p goose --no-default-features --features rustls-tls,local-inference --lib

[sunilkumarvalmiki]

@DOsinga The requested rework is pushed and CI is green now. Could you take another look when you have a chance?