Fix ExternalSessionScanner cache thread safety

[PureWeen]

Replace Dictionary with ConcurrentDictionary for _cache in ExternalSessionScanner. The cache is read/written from a Timer callback (ThreadPool thread) in Scan(), and Dictionary is not thread-safe for concurrent operations.

Found during PR #405 code review — the scanner code is from PR #370.

[PureWeen]

🤖 Multi-Model Consensus Review

CI: ⚠️ No checks configured
Tests: 2792/2794 pass (2 pre-existing flaky WsBridgeIntegrationTests timing failures — unrelated to this change)

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✅ No Findings

5/5 models reviewed this PR and found no bugs. The fix is correct and complete.

Verified:

• All _cache access patterns (TryGetValue, indexer write [key]=, ContainsKey, Keys, TryRemove) are individually atomic on ConcurrentDictionary ✓
• The stale-key eviction pattern (_cache.Keys.Where(...).ToList() → TryRemove) is the standard correct approach — snapshot avoids collection-modified exceptions, TryRemove handles already-gone keys ✓
• The TOCTOU gap (TryGetValue miss → [key]= write) is benign: the timer is one-shot and re-arms only after Scan() completes, so concurrent self-invocation can't occur. Even if it could, last-write-wins with identical values is harmless ✓
• _sessions (the public Sessions property) uses a separate volatile field — correctly handled pre-PR ✓

Recommended Action: ✅ Approve