vMCP: Add per-user caching to discovery manager

[jhrozek]

vMCP: Add per-user caching to discovery manager

Problem

Issue #2502 implements lazy discovery that performs capability aggregation on every request. This works correctly with user authentication but has poor performance since every request queries all backends.

Solution

Add a simple in-memory cache directly to the DefaultManager. Cache keyed by (userID, backend set hash) with hardcoded 5-minute TTL and 1000 entry limit.

Changes

Modify pkg/vmcp/discovery/manager.go:

Add cache fields to DefaultManager:

• Map from cache key (string) to cache entry
• Cache entry contains capabilities and expiration timestamp
• Protect with sync.RWMutex

Update Discover() method:

• Extract user identity from context
• Build cache key from userID and hashed backend references
• Check cache, return if hit and not expired
• On miss, call aggregator and cache result
• Simple eviction: reject caching when at 1000 entry limit

Add cleanup:

• Background goroutine removes expired entries every minute
• Stop() method to terminate goroutine on shutdown

Implementation Notes

Hardcoded values: 5-minute TTL, 1000 entry max size, 1-minute cleanup interval. No configuration needed for MVP.

Cache key stability: Hash backend references after sorting to ensure consistent keys.

Thread safety: Use sync.RWMutex for all cache access.

Call Stop() on manager shutdown to prevent goroutine leaks.