fix: raise clear errors for missing values in panel index

[shuxue6662-a11y]

Description

PanelData.entity_ids / PanelData.time_ids return the raw pandas MultiIndex codes. When the panel index contains missing values (NaN), pandas encodes them as -1, so the ids silently contain negative values and panel_structure_stats() fails later with a generic numpy error:

ValueError: 'list' argument must have no negative elements

This PR makes the failure mode explicit:

• entity_ids and time_ids now raise a ValueError explaining that the panel index contains missing values (NaN), which are not supported.
• panel_structure_stats() now validates that ids are non-negative and raises a clear ValueError naming the statistic and the cause, protecting every call site.

Motivation and Context

Fixes #614.

How Has This Been Tested?

• Added regression tests for missing entity/time index labels and for negative ids in panel_structure_stats.
• pytest linearmodels/tests/panel/test_data.py -> 185 passed.
• black --check, isort --check-only, flake8, and ruff check are clean on the changed files.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.54%. Comparing base (2ce5503) to head (a0a8d36).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #716   +/-   ##
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  Coverage   99.54%   99.54%           
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  Files         101      101           
  Lines       17426    17448   +22     
  Branches     1430     1433    +3     
=======================================
+ Hits        17347    17369   +22     
  Misses         29       29           
  Partials       50       50           

Flag	Coverage Δ
adder	99.53% <100.00%> (+<0.01%)	⬆️
subtractor	99.53% <100.00%> (+<0.01%)	⬆️

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