fix(concurrency): add Python 3.14+ event loop compatibility

[JasperSui]

Purpose

Fix Python 3.14+ compatibility where asyncio.get_event_loop() now raises RuntimeError instead of returning a loop when none exists in the current thread.

Changes

TL;DR

Updated LoopManager to use asyncio.get_running_loop() first, with a graceful fallback to policy-based loop retrieval/creation for Python 3.14+ compatibility. Added comprehensive test coverage for the fallback mechanism.

---

Add compatibility layer for Python 3.14+ where asyncio.get_event_loop()
raises RuntimeError when no event loop exists in the current thread.

• Add _get_or_create_current_loop() method to LoopManager class that
  attempts to get loop via policy, falling back to creating new loop if
  RuntimeError is raised
• Update get_loop() to prioritize asyncio.get_running_loop() for
  'current' strategy, falling back to policy-based approach when no
  running loop exists
• Modify _get_isolated_loop() to use get_event_loop_policy().new_event_loop()
  for consistency with policy-based approach
• Update test_get_loop_current_strategy to mock get_running_loop()
  instead of deprecated get_event_loop()
• Update test_get_loop_isolated_strategy to mock policy instead of
  direct new_event_loop() call
• Add test_get_loop_current_strategy_fallback to verify fallback to
  policy when no running loop exists
• Add test_get_loop_current_strategy_fallback_create_new to verify
  new loop creation when policy raises RuntimeError (Python 3.14+)

Fixes: RuntimeError: There is no current event loop in thread 'MainThread'

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (78e5347) to head (7293c8f).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main      #175   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            9         9           
  Lines          322       326    +4     
  Branches        36        36           
=========================================
+ Hits           322       326    +4     

Files with missing lines	Coverage Δ
src/fastapi_injectable/concurrency.py	100.00% <100.00%> (ø)

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