Fix ReAwaitable to support concurrent await calls (#2108)

[AdrianAcala]

Fix ReAwaitable to support concurrent await calls

This PR adds thread-safe caching with asyncio.Lock to prevent race conditions when multiple tasks await the same ReAwaitable instance concurrently. The implementation maintains backward compatibility for non-asyncio environments.

Changes made:

• Added thread-safe locking mechanism to ReAwaitable class to handle concurrent awaits
• The lock is lazily initialized on first use to maintain compatibility with non-asyncio environments
• Added comprehensive tests for concurrent await scenarios
• Updated documentation to clarify the thread-safe behavior

Why this change is needed:

Previously, when multiple async tasks awaited the same ReAwaitable instance simultaneously, race conditions could occur leading to the underlying coroutine being executed multiple times. This fix ensures that only one task executes the coroutine while others wait and receive the cached result.

Checklist

• [x] I have double checked that there are no unrelated changes in this pull request
• [x] I have created at least one test case for the changes I have made
• [x] I have updated the documentation for the changes I have made
• [x] I have added my changes to the CHANGELOG.md

Related issues

• Closes #2108

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[AdrianAcala]

Sorry. Working on some missing tests.

[AdrianAcala]

This is good to go.

[AdrianAcala]

I pray this passes.

[AdrianAcala]

I used act to run the GitHub actions locally and found there was a bit more errors that I had to fix. I really hope this passes now.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 100.00%. Comparing base (82ef3ef) to head (8cb3088). Report is 338 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##            master     #2150    +/-   ##
==========================================
  Coverage   100.00%   100.00%            
==========================================
  Files           80        81     +1     
  Lines         2485      2586   +101     
  Branches       437        45   -392     
==========================================
+ Hits          2485      2586   +101     

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[AdrianAcala]

I added one more improvement so that it supports multiple async frameworks including asyncio, trio, and anyio, with automatic framework detection and fallback support.