Refactor project dependencies and enhance API documentation

[Allann]

This pull request primarily updates and expands unit tests across several components, with a focus on improving coverage for agent state transitions and decision management. It also updates a project dependency and makes minor test refactorings for clarity and correctness.

Unit test improvements and expansions:

• Refactored and significantly expanded AgentInfoTests to cover the new AgentInfoIdle, AgentInfoRunning, and AgentInfoFailed types, including status transitions, error handling, and immutability via with-expressions.
• Added a comprehensive new test suite for DecisionsService, covering creation, listing, retrieval, resolution, error cases, and round-trip parsing of decisions.

Test correctness and modernization:

• Updated DigestServiceTests to use DateOnly for date parameters and assertions, improving type safety and clarity across multiple test cases. [1] [2] [3] [4] [5] [6] [7]
• Fixed BoardServiceCompleteTaskTests to use the correct SessionStatus enum instead of a string literal for session status.
• Updated AgentServiceTests to assert using the strongly-typed AgentInfoFailed and its Failure property, improving test robustness.
• Modernized LINQ usage in LmStudioExecutorTests by using collection expressions for conciseness.

Dependency updates:

• Bumped the Aspire.AppHost.Sdk version from 13.2.4 to 13.3.0 in the main project file.