fix(dio): support classes that implement Interceptor

[AlexV525]

Closes #2590

Problem

NoSuchMethodError: Class 'X' has no instance method '_invokeError' at runtime when a class uses implements Interceptor instead of extends Interceptor.

PR #2567 added private instance methods (_invokeRequest, _invokeResponse, _invokeError) on Interceptor and dispatched the interceptor pipeline through them. These methods exist only via inheritance — a class that implements Interceptor provides the public onRequest/onResponse/onError methods but does not inherit the private ones, so the pipeline crashes at runtime.

This is a behavioral regression introduced in 5.11.0 (the release that shipped #2567).

Fix

Move the dynamic-dispatch mechanism from private instance methods on Interceptor to a top-level helper function _invokeCallbackDynamically, and call it via closures at each dispatch site (DioMixin.fetch and QueuedInterceptor._handle*). The pipeline now references exclusively the public onRequest/onResponse/onError methods. Both extends Interceptor and implements Interceptor work.

The dynamic invocation is preserved: a Future<void> returned by an async override of a void method is still captured at runtime so _observeInterceptorCallback can observe asynchronous errors — the original motivation from #2567.

Why a top-level function instead of private instance methods?

Private instance methods are not part of the interface contract. Any class using implements Interceptor would not inherit them. A top-level function that takes the public method as a parameter keeps dispatch through the public API surface only. The Interceptor class doc comment now documents this dispatch contract.

Compatibility

• No public API change: no signature change, no symbol added/removed, no typedef change.
• No SDK lower-bound increase.
• No behavior change for extends Interceptor, InterceptorsWrapper, QueuedInterceptor, or QueuedInterceptorsWrapper.
• The fix restores 5.10 behavior for implements Interceptor users.

Verification

Added 3 regression tests covering implements Interceptor:

• Synchronous callbacks through request → response stages.
• Error stage reached on a failing response (404).
• Async onRequest override that throws — dynamic dispatch captures the Future, observer rejects the handler.

dart analyze --fatal-infos clean. 55 tests pass across interceptor_test.dart and queued_interceptor_test.dart (including the existing #2565 deduplication regression test, all #2499 async-hang tests, and wrapper-subclass override tests). Broader non-network suite (dio_mixin_test, cancel_token_test, exception_test): 68 passed, 1 skipped (TLS).

Unverified: Web platform (Chrome/Firefox) not run locally — dynamic dispatch is a Dart language-spec guarantee, not a VM optimization, and the same pattern has been used in _InterceptorWrapperMixin since #2567.

AI Attribution

Implementation, tests, and analysis by GLM-5.2.

[AlexV525]

(Workflows are failing because of flutter/flutter#191056)