assert request params and body in pull_request_test

[juruen]

Context

Follow-up PR to #94 to continue improving our test coverage. These changes assert that the request parameters and body are correctly handled in the pull_request_test file.

[Copilot]

Pull Request Overview

This PR improves test coverage for pull request operations by asserting that the request parameters and request body payloads are correctly handled.

• Updates the HTTP client mock to use handler functions that verify query parameters and request bodies.
• Adds assertions for various endpoints such as listing, merging, updating branches, and creating reviews/PRs.

Comments suppressed due to low confidence (2)

pkg/github/pullrequests_test.go:700

• [nitpick] The test for updating the pull request branch using an empty request body may benefit from an explanatory comment clarifying why an empty body is expected.

expectRequestBody(t, map[string]interface{}{}).andThen(

pkg/github/pullrequests_test.go:1109

• [nitpick] Consider verifying that the explicit use of float64 for the 'position' field is intentional; if the expected type is an integer, using an int literal (with a conversion if necessary) could improve clarity.

"position": float64(10),

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