docs: add recipe for customizing error handling

[Atishay17]

Summary of Changes

Adds a new recipe to docs/user/recipes/ demonstrating two ways to
customize Falcon's error handling behavior:

• Using set_error_serializer to negotiate the error response format
  based on the client's Accept header (JSON vs. plain text).
• Using add_error_handler to intercept a specific exception type and
  either render a custom response directly, or re-raise it as an
  instance of HTTPError so Falcon renders it as usual.

Both examples are provided for WSGI and ASGI, with accompanying tests
in tests/test_recipes.py. Adapted from the example gist linked in
the issue.

Note: drafted with LLM (Claude) assistance for the initial code/docs
and debugging test failures; all code has been run, tested, and
reviewed by me before submission.

Related Issues

Closes #2692

Pull Request Checklist

• Applied changes to both WSGI and ASGI code paths and interfaces (where applicable).
• Added tests for changed code.
• Performed automated tests and code quality checks by running tox.
• Prefixed code comments with GitHub nick and an appropriate prefix.
• Coding style is consistent with the rest of the framework.
• Updated documentation for changed code.
  • Added docstrings for any new classes, functions, or modules.
  • Updated docstrings for any modifications to existing code.
  • Updated both WSGI and ASGI docs (where applicable).
  • Added references to new classes, functions, or modules to the relevant RST file under docs/.
  • Updated all relevant supporting documentation files under docs/.
  • A copyright notice is included at the top of any new modules (using your own name or the name of your organization).
  • Changed/added classes/methods/functions have appropriate versionadded, versionchanged, or deprecated directives.
• Changes (and possible deprecations) have towncrier news fragments under docs/_newsfragments/.
• LLM output, if any, has been carefully reviewed and tested by a human developer.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (9902b95) to head (24d7d40).

Additional details and impacted files

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  Coverage   100.00%   100.00%           
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  Files           64        64           
  Lines         7987      7987           
  Branches      1105      1105           
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  Hits          7987      7987           

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[vytas7]

Hi @Atishay17, and thanks for this PR.
This issue was, OTOH, not looking for contributors -- it was being worked upon in EuroPython '26 Sprints.

Moreover, you marked in the checklist that you ran Tox, but it's not entirely true -- some gates fail.

We will move forward with a version from a human Sprint participant if that gets committed. Otherwise we can circle back on this PR.