fix: runtime bugs and lifecycle gaps from code review

[psachmann]

Summary

Addresses all findings from the fabel code review (FINDINGS.md/REVIEW.md).

High severity

• SetArgs no longer leaks an orphaned Query (with a live background fetch loop) on every cache hit — the losing candidate is now disposed, mirroring PrefetchAsync.
• Cancel() no longer permanently bricks the query — the CancellationTokenSource is recreated under lock instead of being cancelled once and reused forever.
• Value-type TData is now handled correctly: QueryState<TData> tracks presence via explicit HasData/HasLastData flags instead of null-checks, and QueryOptions.InitialData is now an Optional<TData> so default(TData) (e.g. 0) is distinguishable from "not set". Transition.razor uses the explicit flags instead of ??.

Medium severity

• RefetchInterval now routes through Invalidate(), so it respects the subscriber-count and stale-time gates instead of fetching forever with nobody listening.
• Cache entries are now evicted automatically when the last State subscriber leaves (and the pending eviction is cancelled if a new one joins), closing the gap where entries only left the cache via manual Detach().
• QueryCache.Remove now holds its lock for the whole method and disposes any overwritten pending-removal subscription, closing a race with GetOrCreate and a double-Detach leak. Also guards against CacheTime values too large for the scheduler's underlying timer to represent.
• QueryRefreshMonitor.js now exports unregister() to remove its listeners, and both DevTools/RefreshMonitor DisposeAsync methods catch JSDisconnectedException from normal circuit teardown.

Minor cleanup: consistent locking around _lastSuccessAt, disposed-guards on Refetch/Invalidate, suppressed a spurious Idle emission from a superseded fetch, TryAdd in AddDotNetQuery, a cached JsonSerializerOptions instance, a descriptive error on query-key type collisions, an unforgeable QueryKey.Default sentinel, and doc notes on Success/Failure replay semantics.

[codecov]

Codecov Report

❌ Patch coverage is 98.70130% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 94.98%. Comparing base (72a73da) to head (e4548b0).
⚠️ Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
...DotNetQuery.Blazor.DevTools/QueryDevTools.razor.cs	71.42%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #62      +/-   ##
==========================================
+ Coverage   94.56%   94.98%   +0.41%     
==========================================
  Files          21       21              
  Lines         773      877     +104     
  Branches      133      150      +17     
==========================================
+ Hits          731      833     +102     
- Misses         19       21       +2     
  Partials       23       23              

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.