Add explicit PEP 723 script environment setup (preview)

[StellaHuang95]

Fork-review PR for a deliberately limited, default-off feedback rollout.

This replaces the earlier automatic-routing implementation with an explicit setup command. It intentionally does not implement the full roadmap PR9 design.

What this preview does

Adds Python: Set Up Environment for Inline Script (python-envs.setupInlineScriptEnvironment).

The command:

1. uses a supplied local .py URI or the active editor;
2. fails fast when the inline-script preview flag is disabled;
3. requires the open document to be saved;
4. validates saved on-disk PEP 723 metadata;
5. waits for the inline manager's bounded registration readiness;
6. calls InlineScriptEnvManager.create(uri) so current dependencies and requires-python select/reuse the correct cache entry;
7. calls setEnvironment(uri, environment, false) so PR7 persists the per-script association and activates it for the current session without writing project settings.

A failed or cancelled setup leaves the prior association unchanged. Repeated setup is intentional: it lets metadata changes select a new cache entry without adding automatic metadata watchers.

Intentional limitations

• no implicit automatic routing;
• no routing registry or new Memento schema;
• no sidecar provenance or migration machinery;
• no per-script pythonProjects[] registration;
• no CodeLens, picker row, or bulk setup;
• after reload or metadata changes, users may need to invoke setup again;
• feature remains behind python-envs.inlineScripts.enabled.

These limitations keep the first feedback rollout reversible and reduce regression risk.

Size

• production: 108 additions, 0 deletions across four production files
• tests/wiring: focused command and registration coverage

Validation

• TypeScript test compilation
• ESLint
• focused command suite: 15 passing
• four manual review passes; no significant findings remain

[StellaHuang95]

Closing this PR in favor of a clean-history replacement with the same current reviewed file changes.

[StellaHuang95]

Superseded by clean-history replacement #7.