refactor: enable context-aware operations and graceful shutdown throughout

[appleboy]

• Refactor functions throughout the codebase to accept and propagate context.Context for improved cancellation support
• Add signal-based context initialization for graceful shutdown in main
• Move application logic into a run function that returns an exit code for easier control flow
• Use net.ListenConfig with context in place of net.Listen to support context-aware socket binding
• Update browser open, callback server, token exchange, token verification, and refresh operations to be context-aware
• Update all related unit tests to provide context explicitly and use ListenConfig for port binding
• Replace plain lock.release calls with error-ignoring variants to avoid unused return value errors
• Add error handling for JSON encoding failures in all HTTP test handlers
• Introduce a helper function for error condition OAuth device flow tests to reduce duplicate test code
• Minor test improvements including commenting, error messages, and use of constants over literals for tokens

[Copilot]

Pull request overview

This PR refactors the OAuth client to support context-aware operations and graceful shutdown. The changes enable proper cancellation propagation throughout the authentication flows and improve the application's ability to handle interrupts and timeouts gracefully.

Changes:

• Introduce signal-based context initialization in main() with graceful shutdown on SIGINT/SIGTERM
• Refactor main() to return an exit code via a new run() function for cleaner control flow
• Add context.Context parameters to key functions: openBrowser, startCallbackServer, performBrowserFlow, exchangeCode, refreshAccessToken, verifyToken, and checkBrowserAvailability
• Replace net.Listen with net.ListenConfig to support context-aware socket binding
• Update all tests to pass context.Background() and use ListenConfig for port allocation
• Improve error handling by explicitly ignoring lock.release() return values with _ = lock.release()
• Add error handling for JSON encoding in HTTP test handlers
• Introduce pollForTokenErrorTest helper to reduce test code duplication
• Add testAccessToken constant to improve test maintainability

Reviewed changes

Copilot reviewed 11 out of 11 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
main.go	Added signal-based context, refactored main into run() function, propagated context to all operations
tokens.go	Changed lock.release() to explicitly ignore errors with _ = lock.release()
browser.go	Updated openBrowser to use exec.CommandContext for context-aware command execution
browser_flow.go	Added context parameter to performBrowserFlow and exchangeCode, propagated to child operations
callback.go	Added context parameter to startCallbackServer, used ListenConfig for context-aware binding, renamed shadowed variable
detect.go	Added context parameter to checkBrowserAvailability, used ListenConfig for port checking
polling_test.go	Added testAccessToken constant, improved error handling in test handlers, introduced pollForTokenErrorTest helper
main_test.go	Added context.Background() to refreshAccessToken call, added error handling for JSON encoding
filelock_test.go	Updated to explicitly ignore lock.release() errors
detect_test.go	Added context.Background() to all test calls, used ListenConfig for test port allocation
callback_test.go	Added context parameter to startCallbackServerAsync, added gosec nolint directives

Comments suppressed due to low confidence (1)

callback.go:110

• The select statement should respect context cancellation. Currently, it only waits for either a result or a timeout, but doesn't check if the parent context is cancelled. Add a case for context cancellation: case <-ctx.Done(): return "", ctx.Err()

	select {
	case result := <-resultCh:
		if result.Error != "" {
			if result.Desc != "" {
				return "", fmt.Errorf("%s: %s", result.Error, result.Desc)
			}
			return "", fmt.Errorf("%s", result.Error)
		}
		return result.Code, nil

	case <-time.After(callbackTimeout):
		return "", fmt.Errorf("%w after %s", ErrCallbackTimeout, callbackTimeout)
	}

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