Extend MLX backend coverage to ModernBERT and Longformer architectures

[maziyarpanahi]

Description

Adds native MLX token-classification backends for ModernBERT and Longformer, including architecture-specific normalization, converted-weight remapping, rotary/local attention, Longformer global attention, artifact loading, and INT8 shape preservation.

Type of Change

• New feature
• Test addition/improvement

Testing

Validated at exact head 17cd2075ad72d11cb0ac043552035ea949a3c11d against master 2c8c7b13ab757bd205a09899df0d33ca7aa2faa4 on Apple Metal:

• Full repository suite: 13,058 passed, 130 skipped.
• Complete MLX and backend unit suite: 188 passed, 1 skipped.
• Export-matrix coherence suite: 3 passed.
• ModernBERT, Longformer, model registry, and converter mypy checks passed.
• Scoped pre-commit, Ruff, format, Bandit, secret, and diff checks passed.
• Strict documentation artifact build passed.
• Exact staged-artifact Chromium byte-budget gate passed.

Documentation

The MLX export support matrix and public family-resolution errors now include both backends and every supported alias.

Dependencies

No new dependency; tests use the existing optional MLX, PyTorch, and Transformers extras.

Related Issues

Closes #2340
Closes #2345

Screenshots/Examples

Not applicable; parity is covered by deterministic tiny reference models.

[maziyarpanahi]

Maintainer refresh on exact head 6393d30 against master 7c93a8e: the complete Metal-backed MLX suite passed with 166 tests and 1 expected skip; the 42 focused conversion, parity, and FP/INT8 tests passed with no skips; mypy and scoped hooks passed. This branch now consolidates #2340 and #2345; #2481 was closed as superseded. Hosted exact-head checks are starting. No merge performed.

[maziyarpanahi]

Hosted Apple Silicon integration exposed a documentation-coherence failure: the newly supported ModernBERT and Longformer aliases were missing from docs/export-matrix.md. Repaired on exact head 3917459. The focused export-matrix test passes (3 tests), scoped hooks pass, and the strict documentation build passes. Exact-head CI is restarting.