feat(server): automatically activate Python virtual environments

[asfires]

What Changed

• Detects project-local Python virtual environments, including .venv, venv, and uniquely identified non-standard environments.
• Activates the environment automatically for terminal sessions across supported shells and platforms.
• Filters activation bootstrap output and reports concise warnings when activation fails or does not complete.
• Documents terminal virtual environment behavior and worktree handling.
• Adds focused terminal manager coverage for discovery, activation, failures, buffering, and non-Python projects.

Why

Python projects should use their configured virtual environment automatically when opened in T3 Code terminals. This keeps terminal behavior aligned with the project while avoiding incorrect guesses when multiple environments are present.

Checklist

• This PR is small and focused
• I explained what changed and why
• I included before/after screenshots for any UI changes
• I included a video for animation/interaction changes