fix(core): fix callback manager merge mixing handlers (#32028)

[gjeltep]

Description

Fixed BaseCallbackManager.merge() method to correctly preserve the distinction between handlers and inheritable_handlers during merge operations.

Previously, the merge method was using add_handler() which incorrectly added handlers to both lists when inherit=True, causing cross-contamination between regular and inheritable handlers.

The fix directly passes the combined handler lists to the constructor instead of using add_handler(), ensuring proper separation is maintained.

Issue

Fixes #32028

Dependencies

None

Testing

• Modified existing test test_merge_preserves_handler_distinction() to verify handlers remain properly separated after merge

Checklist

• Breaking Changes: No breaking changes - only fixes incorrect behavior
• Type Hints: All functions have complete type annotations
• Tests: Fix is fully tested with existing unit test
• Security: No security implications
• Documentation: No documentation changes needed - bug fix only
• Code Quality: Passes lint and format checks
• Commit Message: Follows Conventional Commits format

[codspeed-hq]

CodSpeed Performance Report

Merging #33617 will improve performances by 11.69%

_{Comparing gjeltep:fix/callback-manager-merge-bug (2b92cbf) with master (e731ba1)}

⚠️ Unknown Walltime execution environment detected

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Summary

⚡ 1 improvement
✅ 12 untouched
⏩ 21 skipped¹

Benchmarks breakdown

	Mode	Benchmark	BASE	HEAD	Change
⚡	WallTime	test_import_time[InMemoryRateLimiter]	198.4 ms	177.7 ms	+11.69%

Footnotes

1. 21 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩