Fix incorrect SignalR caching

[alzimmermsft]

What does this PR do?

Fix incorrect SignalR cache handling where the collected runtimes were never cached, and therefore the cache was being used incorrectly.

GitHub issue number?

[Link to the GitHub issue this PR addresses]

Pre-merge Checklist

• Required for All PRs
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[Copilot]

Pull request overview

Fixes SignalR runtime caching so the actual collected runtime results are cached and subsequent requests can correctly return cached runtimes.

Changes:

• Cache the collected List<Runtime> in SignalRService.GetRuntimeAsync instead of caching the SignalR name.
• Add an Azure.Mcp.Server changelog entry documenting the bug fix.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
tools/Azure.Mcp.Tools.SignalR/src/Services/SignalRService.cs	Corrects cache SetAsync to store the runtime results that GetAsync<List<Runtime>> expects.
servers/Azure.Mcp.Server/changelog-entries/1774535859275.yaml	Adds a “Bugs Fixed” changelog entry for the SignalR caching fix.