feat(fepois): Use dynamic demeaning selection based on backend setting

[RepoBirdBot]

This PR addresses issue #852 by updating the fepois implementation to correctly utilize the specified demeaning backend. Previously, the code was hardcoded to use the Numba backend even when users specified an alternative (e.g., JAX).

Key changes:

• Removed the direct dependency on _set_demeaner_backend and the associated logic that attempted to select a backend-specific demeaning function.
• Updated the call in Fepois so that it directly calls demean, which internally manages the selection based on the provided _fe and mu parameters.
• Simplified the demeaning function call in the ZX_resid computation, ensuring the proper residualization when fixed effects are present.
• Minor docstring adjustments to maintain consistency in section formatting.

These modifications ensure that the demeaning backend is correctly utilized according to the user-specified demeaner_backend setting, providing greater flexibility and correctness when working with different backend implementations.

Please review the changes and run the test suite to ensure that all functionality remains intact.

Closes #852.

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[ariel-frischer]

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This Pull Request was automatically generated by our agent, RepoBirdBot, to address issue #852. This issue was marked as a good first issue and appeared to be unassigned.

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[ariel-frischer]

@RepoBirdBot Update

Here are failed logs you need to fix these:

debug statements (python)................................................Passed
trim trailing whitespace.................................................Passed
fix end of files.........................................................Passed
check yaml...............................................................Passed
check toml...............................................................Passed
check for added large files..............................................Passed
Validate GitHub Workflows................................................Passed
ruff.....................................................................Failed
- hook id: ruff
- files were modified by this hook

Found 6 errors (6 fixed, 0 remaining).

ruff-format..............................................................Passed
nbqa-ruff................................................................Passed
mypy.....................................................................Failed
- hook id: mypy
- exit code: 1

pyfixest/estimation/fepois_.py:154: note: By default the bodies of untyped functions are not checked, consider using --check-untyped-defs  [annotation-unchecked]
pyfixest/estimation/fepois_.py:234: error: "Fepois" has no attribute "demeaner_backend"  [attr-defined]
pyfixest/estimation/feglm_.py:92: note: By default the bodies of untyped functions are not checked, consider using --check-untyped-defs  [annotation-unchecked]
Found 1 error in 1 file (checked 47 source files)