Fix zero-padded integer metadata conflicts

[viranovskaya]

Summary

• compare duplicate filename and sidecar values using the configured integer entity conversion
• treat zero-padded filename entities such as run-01 as equivalent to numeric metadata values such as 1
• preserve conflict detection for booleans, strings, non-integral numbers, and different numeric values

Root cause

Metadata indexing compared the string representation of both values, so the filename value "01" and the JSON number 1 were treated as different even though the run entity is configured as an integer.

Validation

• python -m pytest src/bids/layout/tests/test_layout.py -q — 109 passed
• ruff format --check src/bids/layout/index.py src/bids/layout/tests/test_layout.py
• git diff --check

Fixes #1255