fix(filters): prevent shared Filters instance in concurrent requests

[LisaHusband]

Description

This PR fixes a concurrency-related bug in Flask App Builder’s default API filter handling.
About issue

🐞 Bug Summary

In the default implementation of the FAB API model, Filters instances are often stored as class-level instance variables (e.g., self._filters). Under concurrent request processing—especially in async or multi-threaded environments—this leads to shared mutable state across requests. As a result:

• self._filters is mutated simultaneously by different request handlers;
• Filters from one request may "bleed into" another, returning incorrect or mixed query results;
• No explicit error is raised, making the bug hard to detect;
• Disabling cache does not resolve the issue.

🔧 Fix Summary

To eliminate shared state pollution between concurrent requests:

• The _handle_filters_args method is now refactored to instantiate a new Filters object per request;
• This ensures isolated request-scoped filter logic, avoiding unsafe reuse of self._filters;
• The original self._filters (if any) is preserved for non-query metadata logic like merge_search_filters().

💡 Design Notes

• We avoided using .clear_filters() on the shared instance because it does not guarantee thread safety;
• We chose per-request instantiation rather than object pooling for simplicity and correctness;
• This change prepares FAB for safer use in async-capable Flask environments or threaded WSGI servers.

---

ADDITIONAL INFORMATION

• Is CRUD MVC related.
• Is Auth, RBAC security related.
• Changes the security db schema.
• Introduces new feature
• Removes existing feature

[codecov]

Codecov Report

❌ Patch coverage is 0% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 55.12%. Comparing base (c65e067) to head (f4ea422).
⚠️ Report is 48 commits behind head on master.

Files with missing lines	Patch %	Lines
flask_appbuilder/api/__init__.py	0.00%	3 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           master    #2367       +/-   ##
===========================================
- Coverage   73.81%   55.12%   -18.70%     
===========================================
  Files          72       75        +3     
  Lines        8754     9350      +596     
===========================================
- Hits         6462     5154     -1308     
- Misses       2292     4196     +1904     

Flag	Coverage Δ
python	55.12% <0.00%> (-18.70%)	⬇️

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