use t.Context() instead of context.Background() for test

[catatsuy]

This pull request includes several updates to the Go version and refactors tests to use the testing context. The most important changes include updating the Go version in the workflow and module files, and refactoring multiple test files to replace context.Background() with t.Context().

Go Version Update:

• .github/workflows/go.yml: Updated the Go version from 1.23.x to 1.24.x in the CI workflow.
• go.mod: Updated the Go version from 1.23.0 to 1.24.0.

Test Refactoring:

• internal/cli/cli_test.go: Refactored the TestUploadSnippet function to use t.Context() instead of context.Background(). [1] [2] [3] [4]
• internal/slack/client_test.go: Refactored multiple test functions (TestPostText_Success, TestPostText_Fail, TestPostFile_Success, TestPostFile_FailCallFunc, TestPostFile_FailAPINotOK, TestPostFile_FailAPIStatusOK, TestPostFile_FailBrokenJSON, TestUploadToURL_success, TestUploadToURL_fail, TestCompleteUploadExternal_Success) to use t.Context() instead of context.Background(). [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]
• internal/throttle/exec_test.go: Refactored the TestRun_pipeClose and TestRun_contextDone functions to use t.Context() instead of context.Background(). [1] [2]