feat: add warning handler

[TerryHowe]

What this PR does / why we need it:

Add a warning handler and some tests for it. Gets a little bloat out of options/remote.go

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.36%. Comparing base (d19ca44) to head (3395408).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1856      +/-   ##
==========================================
+ Coverage   85.32%   85.36%   +0.03%     
==========================================
  Files         143      144       +1     
  Lines        6739     6730       -9     
==========================================
- Hits         5750     5745       -5     
+ Misses        703      700       -3     
+ Partials      286      285       -1     

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[Copilot]

Pull Request Overview

This PR introduces a centralized warning handler to manage registry warnings and reduces code duplication. The change extracts warning handling logic from the Remote struct into a dedicated WarningHandler component with proper concurrency handling.

• Adds a new WarningHandler type with thread-safe warning deduplication
• Removes warning handling code from the Remote struct to reduce bloat
• Provides comprehensive test coverage for the new warning handler functionality

Reviewed Changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
cmd/oras/internal/option/warning_handler.go	New warning handler implementation with sync.Map for thread-safe deduplication
cmd/oras/internal/option/warning_handler_test.go	Comprehensive test suite covering deduplication, concurrency, and multi-registry scenarios
cmd/oras/internal/option/remote.go	Removes old warning handling code and integrates new WarningHandler
cmd/oras/internal/option/binary_target.go	Removes manual warning map initialization that's no longer needed

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[Copilot]

Pull Request Overview

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

---

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[Copilot]

Resetting the global singleton in tests can cause issues with test isolation and parallel test execution. Consider refactoring the warning handler to accept an instance parameter or use dependency injection instead of relying on a global singleton.

Suggested change

	oneHandler = warningHandler{}
	var buf bytes.Buffer
	logger := logrus.New()
	logger.SetOutput(&buf)
	logger.SetLevel(logrus.WarnLevel)
	handler := GetHandler(tt.registry, logger)
	var buf bytes.Buffer
	logger := logrus.New()
	logger.SetOutput(&buf)
	logger.SetLevel(logrus.WarnLevel)
	// Use a local handler instance to avoid global state
	localHandler := warningHandler{}
	handler := localHandler.getHandler(tt.registry, logger)

[Copilot]

This test doesn't reset the global singleton state, which could cause interference with other tests. The oneHandler should be reset at the beginning of each test that depends on its state.

[Wwwsylvia]

I found the name warningHandler and warned a bit hard to understand... I don't have a good suggestion though. Is it more like a warner pool or something?