Validate model existence in constructor to fail fast with clear errors

[Copilot]

Model constructor accepted non-existent URLs/IDs without validation, causing confusing errors later during predict() or load_info() calls.

Changes

• Added early validation: _validate_model_exists() makes a GetModel request during __init__ to verify model existence
• Clear error messages: Distinguish URL vs ID-based errors with full context (URL, status, details)
• Performance preservation: Added validate=False parameter used internally in list_versions(), create_version(), etc. to skip redundant checks
• Test coverage: Integration tests for real API validation + mock-based unit tests for validation logic

Example

# Before: confusing error during usage
model = Model(url="https://clarifai.com/user/app/models/nonexistent")
model.predict(...)  # Error: Model not found (unclear where/why)

# After: immediate, clear error at construction
model = Model(url="https://clarifai.com/user/app/models/nonexistent")
# UserError: Model does not exist or cannot be accessed at URL: https://clarifai.com/user/app/models/nonexistent
# Status: Model not found
# Details: The requested model does not exist

Fully backward compatible - validate defaults to True.

Original prompt

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This section details on the original issue you should resolve

<issue_title>Improve error handling for non-existent model URL in Model constructor</issue_title>
<issue_description>The current error handling for the Model constructor does not catch cases where a non-existent model URL is passed. This results in a confusing error message later in the process. The error should be caught earlier and should clearly indicate that the model URL is invalid or the model does not exist.</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Improve error handling for non-existent model URL in Model constructor #825

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