📈 Add telemetry infrastructure, events and documentation

[savannahostrowski]

Closes https://github.com/fastapilabs/cloud/issues/2430 and https://github.com/fastapilabs/cloud/issues/2431
Alrighty, this PR adds anonymous usage telemetry to the extension.

I've added events for:

• Extension lifecycle - Activation, deactivation, and activation failures
• Entrypoint detection - So we can track which detection methods work best (config/pyproject/heuristic)
• Feature usage - Tree view visibility, search execution, CodeLens interactions
• User interactions - Route navigation, route copying, CodeLens clicks (batched)

All extension events are prefixed with extension_

All events automatically include:

• Client type (VS Code Desktop, Cursor, Codespaces, etc.)
• Platform and architecture (darwin/linux/win32, arm64/x64)
• Extension version
• Python and FastAPI versions (from active interpreter if the Python extension for VS Code is activated)
• Session ID
• Workspace metadata (folder count, routes/routers discovered)

Notes about opt-in/out:

• Telemetry is opt-in by default but respects VS Code's global telemetry settings
• We have our own setting for enabling telemetry fastapi.telemetry.enabled
• TELEMETRY.md explains what we collect and why

Tested across VS Code Desktop, GitHub Codespaces and Cursor. Note that telemetry is not tracked in vscode.dev as the posthog library uses Node APIs that are incompatible. I've also set up a dashboard in PostHog.

[tiangolo]

Nice, this looks great! 🚀

I have a couple of questions, but more as questions than suggestions. 🤓

[tiangolo]

Could we also get the versions for:

fastapi-cli, fastapi-cloud-cli, typer, starlette, pydantic?

[savannahostrowski]

100%! Done here! https://github.com/fastapi/fastapi-vscode/pull/22/changes#diff-98d84dbde64a874fc4f2f7e22856cb7a4c9435ea702e4ce98b55de544074473bR13

[tiangolo]

Looks good! 🚀