feat(api): add /validate_prompt endpoint for pre-execution validation

[saurabh-pingale]

Closes #9076

This pull request introduces a new API endpoint, validate_prompt, to validate the structure and integrity of a ComfyUI workflow prompt without executing it.

What does this PR do?

• Adds a POST /validate_prompt endpoint that accepts a JSON workflow prompt.
• The endpoint checks for:
  - Missing required input for all nodes.
  - Invalid values (e.g. ckpt_name that doesn't exist).
  - Incorrectly linked nodes or data types.
• It returns JSON response with "valid": true for a correct prompt or "valid": false along with a node_erros object detailing the specified issues.

Why is this change needed?

Currently, the only way to know if a prompt is valid is to queue it for execution. This is inefficient for both the client and server. This new endpoint provides a quick, lightweight method to pre-validate prompts, improving the user experience and reducing the server load from failed runs.

Testing & Verification
The endpoint has been tested for both valid and invalid workflow scenarios using Postman.

Valid Workflow Response
Below is the Postman screenshot showing successful for a correctly structured workflow. The API returns 200 OK with "valid": true.

Invalid Workflow Response
To ensure robust error handling, an invalid workflow was tested by removing the required positive input from the KSampler node. The API correctly identifies the error, returning a 400 Bad Request with "valid": false and a detailed error message.