Simplify logic for structured outputs across vLLM versions.

This PR refactors the handling of structured output parameters for vLLM generation to improve compatibility across different vLLM versions and simplify the logic for setting structured output options. The main changes focus on unifying the way structured output parameters are imported and used, and on reducing code duplication related to vLLM version checks.

vLLM structured output handling improvements:

• Unified import and usage of StructuredOutputsParams and its key in generation_kwargs based on the vLLM version, ensuring compatibility with both pre- and post-0.10.2 versions.
• Refactored logic for setting the structured output parameter in generation_kwargs, removing duplicated version checks and using a single structured_outputs_key variable for consistency.

Code cleanup:

• Removed redundant imports of vLLM components and version checks from the top-level scope, moving them to where they are needed in the code.
• Updated imports to include LLM, RequestOutput, and SamplingParams only when vLLM is available, improving import hygiene.