✨ Add Test CodeLens navigation

[savannahostrowski]

Needs #3

Closes https://github.com/fastapilabs/cloud/issues/2420

This PR:

• Adds CodeLens navigation above test client calls (e.g., client.get("/items")). Clicking on the lens navigates to the route definition with back-button history preserved. Single matches navigate directly; multiple matches show a peek selection.
• Adds fastapi.showTestCodeLenses setting to disable CodeLens (default: enabled)
• Refactors command registration: uses openLocation for tree view navigation, goToDefinition for CodeLens navigation (where source position matters for history), and keeps goToRouter for router context menu

Review order:

• ✨ Add endpoint discovery service for Endpoint Explorer #2
• ✨ Add file watching for Endpoint Explorer #3
• ✨ Add Test CodeLens navigation #4 ⬅️ (you're here!)