Handle expanding recursive protocol returns

[Rishav23av]

Fixes #21739.

The recursive protocol assumption stack currently closes only when the exact
candidate and protocol instances repeat. That does not happen when a callable
returns a strictly larger instance of its own generic class, so the subtype
check keeps alternating between callable compatibility and protocol
implementation until it raises RecursionError (or segfaults in a compiled
build).

This adds a deliberately narrow cycle closure for the safe case where:

• the exact protocol instance has already been seen with the same candidate class
• there is one non-settable, non-overloaded callable member
• both sides return themselves directly
• the candidate's non-return signature does not depend on its class type variables

Those conditions keep every non-return obligation unchanged while the candidate
return grows. Broader recursive shapes continue through the existing path.

The tests include the reported expanding-return case and a negative case where
the protocol changes from P[int] to P[object] before reaching a fixed point.
The latter ensures the new shortcut does not skip a changing argument
obligation.

Validation:

• python runtests.py self lint
• complete check-protocols.test: 202 passed, 1 skipped
• focused recursive-protocol group: 8 passed
• original reduced CLI reproduction: clean

Disclosure: I developed this change with assistance from Claude Code and Codex.
I reviewed the implementation, challenged several unsafe approaches with
counterexamples, and ran the checks listed above.

[github-actions]

According to mypy_primer, this change doesn't affect type check results on a corpus of open source code. ✅