[SignalR] Fix CTS disposal

[BrennanConroy]

[SignalR] Fix CTS disposal- #69158

Description

When we added cancellation support for non-streaming methods, we didn't dispose the CTS in error cases.

Customer Impact

Fixes a server reliability issue.

Regression?

• Yes
• No

Regressed in 11.0-p6

Risk

• High
• Medium
• Low

Simple change, just missed an error case

Verification

• Manual (required)
• Automated

Packaging changes reviewed?

• Yes
• No
• N/A

[Copilot]

Copilot review overview

🟢 Approval recommended

The change is small, localized, and fixes a concrete CTS disposal leak without altering the normal streaming or cleanup flow.

Review tier: Lite
Findings: None

What changed in this PR

Fixes a CancellationTokenSource lifetime leak in SignalR’s DefaultHubDispatcher streaming invocation path by ensuring the CTS is disposed when an invocation ID collision prevents it from being registered.

Changes:

• Dispose streamCts when ActiveRequestCancellationSources.TryAdd(invocationId, streamCts) fails due to an invocation ID already being in use.
• Minor file-end formatting cleanup.

File	Description
src/​SignalR/​server/​Core/​src/​Internal/​DefaultHubDispatcher.cs	Ensures CTS is disposed on invocation ID collision in StreamAsync to avoid leaking linked CTS instances.

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[dotnet-policy-service]

Hi @BrennanConroy. Please make sure you've updated the PR description to use the Shiproom Template. Also, make sure this PR is not marked as a draft and is ready-to-merge.

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[artl93]

Dispose. Regression. Approved.

[dotnet-policy-service]

Hi @BrennanConroy. This PR was just approved to be included in the upcoming servicing release. Somebody from the @dotnet/aspnet-build team will get it merged when the branches are open. Until then, please make sure all the CI checks pass and the PR is reviewed.