⚡Speed up extension activation

[savannahostrowski]

Closes https://github.com/fastapilabs/cloud/issues/4097

This PR improves extension activation latency by:

• Deferring Python/package version detection off the activation critical path (runs in the background; no longer awaited)
• Parallelizing the discovery file scan (reads candidate .py files concurrently via p-map instead of one-at-a-time)

Note that discovery results are unchanged.

In local testing, the discovery file scan is ~5x faster (1.3s → 0.3s on a 3k-file workspace) and far faster on high-latency filesystems (large monorepos, WSL, etc). Combined with deferring version detection, this targets the activation tail seen in telemetry (p95 ≈ 60s, p99 in minutes).