Backport resourcehealth deserializationfix

[vcolin7]

What does this PR do?

Backport 105990a from PR #2292

GitHub issue number?

#2000

Pre-merge Checklist

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[Copilot]

Pull request overview

Backports the Resource Health fix for Azure API responses that include datetime strings without timezone designators (notably the 0001-01-01T00:00:00 sentinel), preventing System.Text.Json deserialization failures in the resourcehealth tools.

Changes:

• Changed internal Resource Health response models to deserialize datetime fields as string? instead of DateTimeOffset?.
• Added a lenient string? -> DateTimeOffset? parsing helper during internal-to-public model conversion.
• Added unit tests covering mixed timezone/no-timezone datetime formats for service health events and availability status.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
tools/Azure.Mcp.Tools.ResourceHealth/src/Models/Internal/ServiceHealthEventResponse.cs	Switches datetime fields to string? for resilient JSON deserialization.
tools/Azure.Mcp.Tools.ResourceHealth/src/Models/Internal/AvailabilityStatusResponse.cs	Switches datetime fields to string? for resilient JSON deserialization.
tools/Azure.Mcp.Tools.ResourceHealth/src/Models/Internal/ResponseModelExtensions.cs	Adds lenient datetime parsing when mapping internal responses to public models.
tools/Azure.Mcp.Tools.ResourceHealth/tests/Azure.Mcp.Tools.ResourceHealth.UnitTests/ServiceHealthEvents/ServiceHealthEventDeserializationTests.cs	Adds coverage for service health event datetime formats (with/without timezone, nulls, mixed).
tools/Azure.Mcp.Tools.ResourceHealth/tests/Azure.Mcp.Tools.ResourceHealth.UnitTests/AvailabilityStatus/AvailabilityStatusDeserializationTests.cs	Adds coverage for availability status datetime formats (with/without timezone, nulls, single vs list).