Use claude-haiku-4-5 for API validation instead of deprecated claude-3-5-haiku-20241022

[Copilot]

validate_api_access() hardcoded claude-3-5-haiku-20241022 which is deprecated, causing all review actions to fail. The method intentionally uses a Haiku model (cheap/fast) for validation pings rather than the configured analysis model.

• Added VALIDATION_MODEL constant in constants.py set to claude-haiku-4-5
• Updated validate_api_access() to use VALIDATION_MODEL instead of the hardcoded deprecated string
• Updated tests to verify validation uses the dedicated constant, not self.model

# Before — hardcoded deprecated model
self.client.messages.create(model="claude-3-5-haiku-20241022", ...)

# After — dedicated constant for validation pings
self.client.messages.create(model=VALIDATION_MODEL, ...)

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