fix(openai): apply request_params to outgoing API payload

[TurboWindX]

Problem

request_params is defined in ModelConfig and documented as supporting
provider-specific parameters (e.g. thinking, extra_body fields), but
create_request() in formats/openai.rs never reads it. Any params set
in config are silently discarded before the request is sent.

This breaks use cases like disabling DeepSeek V4 thinking mode:

request_params:
  thinking:
    type: disabled
The payload goes out without the field, causing a 400: reasoning_content must be passed back error on subsequent
turns.

Fix

Merge request_params into the payload just before return in
create_request(). Added a test covering both the merge and field
override behaviour.

Testing

cargo test -p goose --lib -- test_create_request_merges_request_params
test result: ok. 1 passed

Fixes #9083
Related to #904

[DOsinga]

LGTM — the fix correctly merges request_params into the OpenAI format payload, matching the existing pattern in databricks.rs. Dropped the test and merged main.

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