[Misc] Further clean up chunked prefill and prefix caching init

[DarkLight1337]

Purpose

• Add warning message if chunked prefill is disabled for generation models
• Update type annotation of CacheConfig.enable_prefix_caching to exclude None values

Test Plan

Test Result

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• The purpose of the PR, such as "Fix some issue (link existing issues this PR will resolve)".
• The test plan, such as providing test command.
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[chatgpt-codex-connector]

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[gemini-code-assist]

Code Review

This pull request provides some good cleanups. It changes enable_prefix_caching in CacheConfig to be a bool that defaults to True, which clarifies the configuration, aligns with the likely original intent, and allows removing a redundant bool() cast. It also adds a helpful warning when chunked_prefill is disabled on models where it's the default. These changes improve code clarity and user experience. The implementation looks correct.

[yewentao256]

Thanks for the work!