requirements.txt: replace pinned transformers==4.50.0 with lower bound

[smirnovlad]

Problem

The main branch pins transformers==4.50.0 in requirements.txt. This is problematic for downstream projects (e.g. ThinkBooster) and users who need newer transformers versions for new model architectures or vLLM compatibility.

An exact pin in a library forces every consumer to use that exact version, which:

• Breaks when combined with packages that need newer transformers (e.g. vLLM requires >=4.56.0)
• Prevents users from getting bug fixes and new model support
• Creates unnecessary dependency conflicts

Current state

Branch	Constraint
main	transformers==4.50.0
dev	transformers>=4.48.0

The dev branch already uses a lower bound (>=4.48.0) and has been running without issues for 2+ months across ThinkBooster's full experiment suite with transformers 4.56+.

Action item

Replace the exact pin with a lower bound in requirements.txt on main:

- transformers==4.50.0
+ transformers>=4.48.0

This is already resolved in PR #436 (dev → main merge), but filing this issue for tracking in case the PR takes time to merge.

Context

• PR Merge dev into main: vLLM integration, API uncertainty, hidden states capture #436 merges dev → main and includes this fix
• ThinkBooster dependency analysis: PyPI package publishing roadmap thinkbooster#239 (comment)
• The relaxation was originally done by @rediska01234 in commit cc46e0e on the dev branch as part of vLLM+UHead integration