Add support for workflows run id

[mtmk]

The Workflow Dispatch API can now return the Run ID:

https://github.blog/changelog/2026-02-19-workflow-dispatch-api-now-returns-run-ids/

• Add DispatchesPostResponse model
• Add unit test for DispatchesRequestBuilder

[Copilot]

Pull request overview

This PR updates the GitHub Actions workflow dispatch client to support the new API behavior where workflow_dispatch can return run details (including a run ID) when return_run_details is enabled.

Changes:

• Introduces a new DispatchesPostResponse model to represent the run details response payload.
• Updates DispatchesRequestBuilder.PostAsync to return DispatchesPostResponse instead of Task/no-content behavior.
• Adds unit tests validating PostAsync returns the expected response and still throws on null body.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
test/Dispatches/DispatchesRequestBuilderTest.cs	Adds tests for PostAsync returning a parsed response payload (run id + URLs) and null-body validation.
src/GitHub/Repos/Item/Item/Actions/Workflows/Item/Dispatches/DispatchesRequestBuilder.cs	Changes PostAsync from “no content” to deserializing and returning a DispatchesPostResponse.
src/GitHub/Repos/Item/Item/Actions/Workflows/Item/Dispatches/DispatchesPostResponse.cs	Adds the response model for workflow_dispatch when return_run_details is true.

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