✨ Add endpoint discovery service for Endpoint Explorer

[savannahostrowski]

Closes https://github.com/fastapilabs/cloud/issues/2418 and https://github.com/fastapilabs/cloud/issues/2452

Apologies for the fairly large PR - it's a fairly large component (it's basically the guts of the extension) and I wanted the end result to be something we could play with as a team. The PR is really about ~1700 lines of implementation and then a whole lot of tests/fixtures.

This PR:

• Adds a new endpoint discovery service that parses the program's AST to extract FastAPI routes, routers, etc. and resolves imports to follow router chains across files
• Updates the tree view to group routers by declared prefixes, creates synthetic groups when multiple routes share the same prefix segment, creates display labels (prefix, tag, file in that priority order), and shows the number of routes per router

Key files to review
Please prioritize 🙏:

• src/core/routerResolver.ts - Main entry point, builds the router graph by following imports
• src/core/importResolver.ts - Resolves Python imports (relative, absolute, re-exports). This is also probably the most complicated piece IMO.
• src/core/transformer.ts - Converts router graph to tree view structure
• src/core/extractors/ - Individual extractors for decorators, routers, imports, etc.

At this point, users can:

• Expand/collapse all routes to see/hide routes in the app
• Right-click to copy full path/prefix
• Click a route to navigate to it
• Click refresh to re-scan the workspace (file watching is up next!)
• Exposes a setting to allow a user to specify an app entry point outside of what's currently supported - see defaultPatterns in extension.ts

Other considerations

• I considered using ast-grep for this but this would mean that we couldn't distribute a single VSIX anymore since it imports a native module (~7MB per platform vs ~350KB WASM that works everywhere).
• I want to add proper logging in a separate PR via https://github.com/fastapilabs/cloud/issues/2527

Review order:

• ✨ Add endpoint discovery service for Endpoint Explorer #2 ⬅️ (you're here!)
• ✨ Add file watching for Endpoint Explorer #3
• ✨ Add Test CodeLens navigation #8

[savannahostrowski]

TIL https://github.com/orgs/community/discussions/23978 😭

[buurro]

Tested locally and works great!

Found a bug, but it will be handled on a separate PR

[savannahostrowski]

I believe this is the problematic bit for #7