[Python/Beam] Add MailboxProcessor tests and fix bugs

[dbrattli]

Summary

• Python: Convert mailbox_processor.py to PEP 695 type parameter syntax and add missing post_and_async_reply standalone function
• Python: Create dedicated TestMailboxProcessor.fs with 13 comprehensive tests (moved out of TestAsync.fs)
• Beam: Fix fable_async_builder:for/2 to handle seq/enumerable objects in async for-in loops (not just plain lists)
• Beam: Fix fable_mailbox.erl cancellation — token was accepted but ignored; now stored in state and checked in process_events
• Beam: Add 3 loop tests to LoopTests.fs isolating async for-in with negative step ranges

Note: No external changes for Python, everything as before. Only rewriting of fable-library. Fix for Beam, but beam has not been released yet so no need for a ChangeLog entry. Nothing changed.

Test plan

• All 13 Python MailboxProcessor tests pass
• All 2138 Beam tests pass (including new MailboxProcessor and loop tests)
• Tests cover: basic post, postAndAsyncReply, falsy values, FIFO ordering, state accumulation, static/instance Start, buffering, error handling, cancellation, reply pairing, deep recursion, stateful replies

🤖 Generated with Claude Code

[github-actions]

Python Type Checking Results (Pyright)

Metric	Value
Total errors	28
Files with errors	5
Excluded files	5
New errors	✅ No

Excluded files with errors (5 files)

These files have known type errors and are excluded from CI. Remove from pyrightconfig.ci.json as errors are fixed.

File	Errors	Status
temp/tests/Python/test_applicative.py	12	Excluded
temp/tests/Python/test_mailbox_processor.py	10	Excluded
temp/tests/Python/test_hash_set.py	3	Excluded
temp/tests/Python/test_nested_and_recursive_pattern.py	2	Excluded
temp/tests/Python/fable_modules/thoth_json_python/encode.py	1	Excluded