Fix race condition in discovery manager causing duplicate aggregations

[therealnb]

Summary

This PR fixes a race condition in the discovery manager that causes duplicate capability aggregations when multiple concurrent requests arrive simultaneously at startup.

Problem

Without this fix, when multiple requests arrive concurrently:

1. Each request checks the cache (all miss)
2. Each request triggers expensive capability aggregation
3. This results in duplicate work and "recursive-looking duplicate 'Starting capability aggregation' logs"

This is a distinct issue from the race condition fixed in #3450.

Solution

Uses singleflight.Group to deduplicate concurrent capability aggregation requests:

• Multiple concurrent requests for the same cache key wait for a single aggregation
• The first request performs the work, subsequent requests reuse the result
• Includes a double-check of the cache after acquiring the singleflight lock
• Prevents wasted compute and confusing duplicate logs

Changes

• Added singleflight.Group field to DefaultManager
• Modified Discover() method to wrap aggregation in singleFlight.Do()
• Added double-check cache pattern after acquiring singleflight lock

Testing

Existing tests pass. The race condition is most visible under high concurrency at startup when multiple clients connect simultaneously.

[codecov]

Codecov Report

❌ Patch coverage is 72.72727% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 64.93%. Comparing base (2acfcfc) to head (8d92893).

Files with missing lines	Patch %	Lines
pkg/vmcp/discovery/manager.go	72.72%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3471      +/-   ##
==========================================
- Coverage   64.95%   64.93%   -0.02%     
==========================================
  Files         396      396              
  Lines       38492    38499       +7     
==========================================
- Hits        25001    25000       -1     
- Misses      11542    11550       +8     
  Partials     1949     1949              

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