[Misc] Use VLLMValidationError consistently in completion and chat completion protocol

[umut-polat]

Purpose

Replace remaining ValueError raises with VLLMValidationError in the
model validators for CompletionRequest and ChatCompletionRequest.

This follows the same pattern established in the responses protocol and
SamplingParams, where VLLMValidationError provides structured error
context (parameter, value) for better client-side error handling.

Changes

vllm/entrypoints/openai/completion/protocol.py

• validate_prompt_and_prompt_embeds: ValueError → VLLMValidationError
• check_cache_salt_support: ValueError → VLLMValidationError (with parameter/value)

vllm/entrypoints/openai/chat_completion/protocol.py

• check_structured_outputs_count: 2× ValueError → VLLMValidationError
• check_tool_usage: 6× ValueError → VLLMValidationError (with parameter/value)
• check_generation_prompt: ValueError → VLLMValidationError
• check_cache_salt_support: ValueError → VLLMValidationError (with parameter/value)

Test Plan

Existing tests cover these validators. The change only affects the
exception type raised (VLLMValidationError extends ValueError), so all
existing error handling continues to work.

[gemini-code-assist]

Code Review

This pull request does a good job of consistently using VLLMValidationError instead of ValueError for request validation, which improves error reporting for clients. I've identified a couple of instances where the new validation errors could be made even more informative by including the parameter argument, aligning them more closely with the other changes in this PR. My suggestions aim to enhance this consistency.

[gemini-code-assist]

For consistency with the other changes in this PR, it would be better to specify which parameter is causing the validation error. Since this is a conflict between two parameters, you can pick one to report. add_generation_prompt seems like a reasonable choice.

Suggested change

	raise VLLMValidationError(
	"Cannot set both `continue_final_message` "
	"and `add_generation_prompt` to True.",
	)
	raise VLLMValidationError(
	"Cannot set both `continue_final_message` "
	"and `add_generation_prompt` to True.",
	parameter="add_generation_prompt",
	)

[gemini-code-assist]

For consistency with the goal of this PR to provide structured error context, it's a good idea to include the parameter that failed validation. In this case, since either prompt or prompt_embeds is required, you can specify prompt as the parameter.

Suggested change

	raise VLLMValidationError(
	"Either prompt or prompt_embeds must be "
	"provided and non-empty.",
	)
	raise VLLMValidationError(
	"Either prompt or prompt_embeds must be "
	"provided and non-empty.",
	parameter="prompt",
	)

[mergify]

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[robertgshaw2-redhat]

how does this change impact user-facing status codes/

[umut-polat]

No change in status codes — ValueError and VLLMValidationError both return 400 already (create_error_response handles both as BadRequestError). The actual difference is the param field in the JSON error body: VLLMValidationError populates it (e.g. "param": "max_tokens"), plain ValueError leaves it null. So clients get better diagnostics without any breaking change.

[DarkLight1337]

The parameter here should be tools

[DarkLight1337]

Actually, this PR looks like a duplicate of #36254

[umut-polat]

closing in favor of #36254 which covers the same files. will rebase that one if needed.