Feat Exception Handlers

[NEFORCEO]

🚀 Description

Adds FastAPI-style exception_handler decorators to FastHTTP, Router, and EventHooks. When a route raises an exception matching a registered type (or one of its subclasses, resolved via MRO), the handler runs and its return value becomes the route's result instead of the request simply failing.

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🧩 Type of Change

Please select one:

• feat: New feature
• fix: Bug fix
• docs: Documentation update
• refactor: Code refactoring
• ci: CI/CD changes
• chore: Maintenance

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✅ Checklist

• Tests added or updated
• Documentation updated
• No breaking changes
• Code follows project style

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🔗 Related Issues

Fixes #

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📸 Additional Context

• New ExceptionHandler type and exception_handler/get_exception_handler API in fasthttp/events.py.
• HTTPClient._run_exception_handler invokes the matching handler and processes its result like a normal handler result.
• Exposed via FastHTTP.exception_handler and Router.exception_handler.
• Docs updated in docs/en/middleware.md, docs/en/reference/fasthttp.md, and Russian equivalents.
• Tests added in tests/test_app.py and tests/test_client.py.

[github-actions]

👋 Thanks for your contribution, @NEFORCEO!

Please make sure:

• Code is clean and formatted
• Tests are passing
• PR description is clear

Maintainers will review it soon.

[codspeed-hq]

Merging this PR will improve performance by 41.43%

⚠️ Different runtime environments detected

Some benchmarks with significant performance changes were compared across different runtime environments,
which may affect the accuracy of the results.

Open the report in CodSpeed to investigate

⚡ 6 improved benchmarks
✅ 6 untouched benchmarks

Performance Changes

	Benchmark	BASE	HEAD	Efficiency
⚡	test_response_req_text_from_json	213.2 µs	134.5 µs	+58.55%
⚡	test_response_repr	196.4 µs	133.9 µs	+46.76%
⚡	test_response_property_access	194.5 µs	133.3 µs	+45.91%
⚡	test_response_creation	201.3 µs	139.3 µs	+44.54%
⚡	test_response_json_parsing	205.6 µs	143.4 µs	+43.38%
⚡	test_middleware_after_response	403.1 µs	354.4 µs	+13.73%

Tip

Curious why this is faster? Comment @codspeedbot explain why this is faster on this PR, or directly use the CodSpeed MCP with your agent.

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_{Comparing feat/exception-handlers (a74b53c) with master (6f3d441)¹}

Footnotes

1. No successful run was found on master (53d8dbd) during the generation of this report, so 6f3d441 was used instead as the comparison base. There might be some changes unrelated to this pull request in this report. ↩