Stop the dev server being interrupted at startup, and make interpreter selection work from .pyxl files

[shivamsn97]

The issue

Two problems, both reported from a real session and both rooted in how the extension talks to the Python environment.

1. The dev server was killed seconds after it started. Using "Debug Pyxle app (React browser)", the server came up and was then immediately interrupted:

pyxle dev
✅ Pyxle dev server ready in 1690 ms
^C⚠ Interrupt received; stopping dev server
pyenv shell 3.14.4

The extension started the server by opening a shell terminal and typing pyxle dev into it. A shell terminal is writable by anything, and the Python extension activates the selected environment in every newly created one. Modern activation goes through TerminalShellIntegration.executeCommand(), whose documented behaviour is to send ^C first to interrupt any running command — so VS Code interrupted the just-started server and typed the activation line (pyenv shell …, source .venv/bin/activate) in its place. Nothing the extension could time around: activation is asynchronous and lands whenever the interpreter resolves.

2. "Debugging needs pyxle-framework 0.8.0 or newer" with 0.8.0 installed. Two causes:

• The pre-launch check ran against VS Code's selected interpreter, but the React-browser flow launched whatever pyxle was first on the shell PATH — routinely a different environment. The two configurations could run two different installs of two different versions.
• The message offered only Copy upgrade command. When the working pyxle lives in another environment — the common case — upgrading the selected one is the wrong advice, and there was no way to switch environments from the dialog. There is also no interpreter indicator in a .pyxl editor (the Python extension scopes its own to .py files), so the interpreter driving the launch was invisible and unchangeable without opening an unrelated Python file.

The fix

The extension owns the dev-server process. A Pseudoterminal spawns it directly instead of typing into a shell. No shell means terminal.shellIntegration is never populated, so the ^C-sending API cannot be called on it, and a foreign Terminal.sendText() is delivered to our handleInput, where it is dropped — the injection is inert by construction rather than merely unlikely. The panel still shows the server's output, Ctrl-C still stops it, and shutdown is now exact: SIGINT to the process group (so Vite and the SSR workers go too), a SIGKILL backstop, taskkill /T /F on Windows.

Both configurations run the same environment. The React-browser flow now launches <selected interpreter> -m pyxle dev through the same argv helper and the same pre-launch check as the Python flow.

The pre-launch check is capability-based and no longer a dead end.

• It asks what the interpreter can do (find_spec('pyxle.__main__')), never what its package metadata claims — so an editable install whose dist-info still reads 0.7.5 while the code is current launches normally. The reported version is shown for context only, labelled as package metadata.
• A probe that cannot answer (interpreter crash, timeout, spawn failure) reports unknown and lets the launch proceed, so the debugger surfaces the real error instead of the check asserting "pyxle is not installed".
• Both failure dialogs lead with Select Interpreter, always open the picker when clicked, and re-probe afterwards — so fixing the environment (or picking another one) continues the launch instead of costing another F5.

New: the interpreter is visible and switchable from .pyxl files — a status-bar item showing the selected environment (click to change), plus a Pyxle: Select Python Interpreter command.

Also in this change: owned dev servers are stopped when VS Code shuts down (they run in their own process group and would otherwise keep holding their ports), and command palette entries no longer read "Pyxle: Pyxle: …".

Verification

• tsc --noEmit clean; npm test green (23 tests, 12 new covering the probe, version parsing and interpreter labelling).
• Probed against three real environments: an editable install reporting 0.7.5 with current code → launches (previously refused), a 0.8.0 install → ok, a nonexistent interpreter → unknown rather than a false "not installed".
• Packaged and installed the VSIX locally; the built bundle contains no sendText("pyxle dev").

The changelog entry is under Unreleased — this ships with the next extension release.

[shivamsn97]

Reviewed and merging. The diagnosis is right and the pty approach is the correct fix rather than a timing workaround: VS Code routes Terminal.sendText() to Pseudoterminal.handleInput and never populates shellIntegration on an extension terminal, so the ^C-sending activation path is structurally unreachable. argv stays an array throughout, so interpreter paths with spaces are safe on both platforms, and the 0.3.0 invariants (refcounted ownership, discovery startedAt matching, provide-only Dynamic registration) are intact.

Five lifecycle gaps in the new code are fixed in a follow-up commit on this branch:

1. Windows teardown could throw and strand the tree. cp.spawn reports a missing/blocked taskkill asynchronously as an error event — the surrounding try/catch cannot see it, and an unhandled error throws. Now has a listener that falls back to killing the leader, instead of relying on a SIGKILL timer that only reaches that leader and leaves Vite holding the port.
2. Settled on both exit and close. exit fires before piped stdio drains, so a startup failure printed "The dev server stopped" above its own traceback, and a deliberate stop closed the panel over output still in flight. Settles on close alone now — verified against Node that a nonexistent command emits error → close and never exit, so the case the pair was guarding is still covered.
3. onDidExit was dead code. The React launch now subscribes to it, so a server that dies early cancels the readiness wait instead of holding browserLaunchInFlight for the full 120 s and silently swallowing every further F5 — which the new unknown probe verdict makes more reachable, by design.
4. Discovery was not re-read after the interpreter gate, which can block on a dialog and a quick pick for an unbounded time. A second pyxle dev could be spawned onto a port that had since been taken; it re-checks and re-adopts now.
5. Changelog accuracy — detached gives one signal the whole tree but removes the SIGHUP a pty close used to deliver, so a force-quit can still leave the server running. The entry says so rather than claiming shutdown always stops it.

tsc --noEmit clean, 23/23 tests pass, packaged cleanly, and the bundle contains no sendText("pyxle dev").