[FEATURE]: Built-in flag-gated Ruff LSP server (parity with the existing ty LSP)

[natori-hrj]

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Describe the enhancement you want to request

Follow-up to #5345, which was auto-closed for inactivity (the bot suggested opening a fresh one).

opencode already ships ty as a built-in Python LSP behind an experimental flag (experimentalLspTy in packages/opencode/src/lsp/server.ts), but there is no built-in ruff server counterpart — Ruff is wired up only as a formatter. Astral's own guidance is to run ty + ruff together (ty for types, ruff for lint + quick-fixes / import fixing), so the built-in Python LSP support is currently half-complete.

Proposal: add a flag-gated ruff LSP that mirrors the existing Ty server exactly and runs alongside pyright/ty rather than replacing them (same way eslint runs next to typescript):

• new Ruff entry in lsp/server.ts, extensions .py/.pyi, launched via ruff server
• gated by an experimental flag, symmetric with ty
• detect ruff on PATH or in .venv/venv, no auto-install (per the earlier maintainer note in [FEATURE]: ty + ruff lsp rather than pyright #5345)

I know this is achievable today via custom lsp config, and there were prior stale PRs (#9126, #14850). Filing this mainly to confirm whether a first-class, ty-symmetric built-in is wanted before I open a PR — happy to implement it if so.