feat(python-setup): route the setup command to the uv flow when active

[rugpanov]

Why

When the uv-native Python setup is the active surface for a project — the environment.pythonSetup opt-in plus a uv-suitable project, the same gate the configuration view already dispatches on — the databricks.environment.setup command still ran the legacy checklist. Every "set up my Python env" trigger funnels through that command:

• the Databricks Connect status bar button,
• the config-view legacy rows,
• the command palette entry,
• the Run/Debug current file gate's own executeCommand("databricks.environment.setup").

So all of them drove the wrong flow for uv projects. Separately, the legacy interpreter-change listener would pop a second install prompt on top of the uv flow.

What

• pythonSetupRouting.ts — a narrow UvPythonSetup seam plus routeEnvironmentSetup, which sends databricks.environment.setup to the uv flow when it is visible, else the legacy checklist. One change reroutes every trigger surface (the palette command is not hidden — it routes).
• Interpreter-change guard (EnvironmentDependenciesVerifier) — suppress the legacy auto-install prompt when uv is active, so the two flows don't both prompt.
• extension.ts — wire both.

On readiness (deliberate design choice)

The run/debug gate and the status bar keep reading the legacy environment.dependencies check for readiness. That check reads real installed state (interpreter version + databricks-connect presence/version), so it is reload-safe and interpreter-safe — unlike the uv flow's session-only ready flag, which is empty after a window reload and is never invalidated when the user switches interpreters. Routing only the setup trigger (not the readiness signal) drives the uv setup from every surface while avoiding a reload / interpreter-staleness regression. No cross-fallback: when uv is active it is the only setup path.

All new dependencies are optional and default to legacy behavior, so nothing changes for users who have not opted in (the uv gate returns false → every surface behaves exactly as before). No new settings, persisted state, telemetry events, or when-clause flags.

Verification

• Unit tests for the command routing and the interpreter-change guard.
• yarn test:unit: 790 passing, 0 failing (10 pre-existing pending).
• eslint + prettier -c clean on all touched files.
• Reviewed with Isaac Review (0 findings), Codex, a Claude code-review agent, a devil's-advocate pass, and a CODE_CONVENTIONS.md compliance pass; this design incorporates their feedback (the earlier readiness-routing approach was dropped after review flagged the reload/interpreter regressions).

This pull request and its description were written by Isaac.

[rugpanov]

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[rugpanov]

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[rugpanov]

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• PR number: 2118
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