Support controller reuse across multiple NinjaExtraAPI instances

[jdiego]

Closes #287

Updated NinjaExtraAPI.register_controllers to handle per-API isolation:

• If a controller is already registered globally, create a cloned subclass
  and a fresh APIController to avoid shared global state.
• Added _registered_controllers (WeakSet) to guard against duplicate registration on the same API.
• Added _controller_clones (dict) to cache per-API clones and prevent creating multiple subclasses for the same original controller.
• Ensure API instance is bound before building routers so PathViews register under the correct API context.

Enhanced tests:

• Verified controllers bind operations to the correct API.
• Ensured auto_discover_controllers respects auto_import=False.
• Asserted duplicate registration on the same API is a no-op.
• Verified same controller can be mounted on multiple APIs with isolation
  (distinct routers, correct self.api binding).
• Confirmed OpenAPI schemas are consistent across multiple APIs.
• Added regression test for clone caching to prevent duplicate clones.

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.87%. Comparing base (6a592c1) to head (6dbc837).
⚠️ Report is 59 commits behind head on master.
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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #293      +/-   ##
==========================================
- Coverage   98.01%   97.87%   -0.14%     
==========================================
  Files          68       67       -1     
  Lines        2770     2824      +54     
==========================================
+ Hits         2715     2764      +49     
- Misses         55       60       +5     

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[eadwinCode]

@jdiego Nice work here. Thanks.
I think we need to a add an INFO log when we are cloning the controllers for another API instance. This is important, so that devs are aware that the controller was cloned and they want this set of behaviours. Also, it will be good for debugging

[jdiego]

@eadwinCode done.