fix(BA-4108): Use model_validate for health check info to apply Pydantic defaults

[seedspirit]

resolves #8354 (BA-4108)

Checklist: (if applicable)

• Milestone metadata specifying the target backport version
• Mention to the original issue
• Installer updates including:
  • Fixtures for db schema changes
  • New mandatory config options
• Update of end-to-end CLI integration tests in ai.backend.test
• API server-client counterparts (e.g., manager API -> client SDK)
• Test case(s) to:
  • Demonstrate the difference of before/after
  • Demonstrate the flow of abstract/conceptual models with a concrete implementation
• Documentation
  • Contents in the docs directory
  • docstrings in public interfaces and type annotations

[Copilot]

Pull request overview

This PR fixes a bug where the initial_delay field in health check configurations was being passed as None when missing from YAML, preventing Pydantic from applying its default value of 60.0. The fix replaces manual object construction with Pydantic's model_validate method, which properly applies field defaults.

Changes:

• Replaced manual ModelHealthCheck construction with model_validate() to properly handle missing optional fields
• Added comprehensive test coverage for health check configurations with various field combinations
• Updated changelog to document the fix

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
src/ai/backend/manager/registry.py	Replaced manual construction of ModelHealthCheck with model_validate() to properly apply Pydantic defaults for missing fields
tests/unit/manager/test_registry.py	Added comprehensive test suite for health check configurations, including test cases for missing initial_delay field and other optional fields
changes/8389.fix.md	Added changelog entry documenting the fix

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[Copilot]

The description says "when initial_delay field is nullable" but this is inaccurate. The issue is that initial_delay is NOT nullable (it's typed as float, not float | None), which is why passing None prevented Pydantic from using its default value. Consider rephrasing to: "preventing pydantic validation error when initial_delay field is missing from the YAML"

Suggested change

	Use `model_validate` for health check info to apply Pydantic defaults, preventing pydantic validation error when `initial_delay` field is nullable
	Use `model_validate` for health check info to apply Pydantic defaults, preventing pydantic validation error when `initial_delay` field is missing from the YAML