Fix BetterTransformer compatibility with transformers >= 4.49

[AimoreRRD]

Summary

• Handle RuntimeError when importing BetterTransformer with transformers >= 4.49 • Gracefully disable BetterTransformer optimization when unavailable due to version incompatibility • Maintain backward compatibility with older transformers versions

Problem

BetterTransformer from optimum v1.27.0 is deprecated and incompatible with transformers >= 4.49, causing a RuntimeError that prevents infinity_emb from starting.

Solution

• Wrap BetterTransformer imports in try-except blocks
• Add BETTERTRANSFORMER_AVAILABLE flag to track availability
• Skip BetterTransformer optimization when not available, logging info message
• Continue normal operation without optimization

Test plan

• [x] Test with transformers 4.53.3 and optimum 1.27.0
• [x] Verify infinity_emb starts successfully with Qwen3 models
• [x] Confirm graceful fallback when BetterTransformer unavailable
• [x] Check that existing functionality remains unchanged

[AimoreRRD]

Fixes #642