feat: complete async LINQ materialiser layer for IBLiteQueryable

[Copilot]

IBLiteQueryable<T> was missing async counterparts for most common LINQ terminators, forcing users to fall back to sync (blocking) .First(), .Last(), .Min(), etc.

New extensions (BLiteQueryableExtensions.cs)

All follow the existing pattern — cast to IAsyncEnumerable<T> at runtime, materialise manually, no System.Linq.Async dependency.

Extension	Overloads	Empty-sequence behaviour
FirstAsync	plain + predicate	throws InvalidOperationException
SingleAsync	plain + predicate	throws on empty or multi-element
LastAsync	plain + predicate	throws InvalidOperationException
LastOrDefaultAsync	plain + predicate	returns default
ElementAtAsync	index	throws ArgumentOutOfRangeException
ElementAtOrDefaultAsync	index	returns default
MinAsync<T,TResult>	selector	throws on empty
MaxAsync<T,TResult>	selector	throws on empty
SumAsync	int / long / double / decimal	returns zero
AverageAsync	int→double / long→double / double / decimal	throws on empty
ForEachAsync	Action<T> + CancellationToken	no-op

// Before: silently blocks
var first = collection.AsQueryable().Where(x => x.Active).First();

// After: fully async
var first   = await collection.AsQueryable().Where(x => x.Active).FirstAsync(ct);
var last    = await collection.AsQueryable().OrderBy(x => x.Ts).LastAsync(ct);
var total   = await collection.AsQueryable().SumAsync(x => x.Amount, ct);
var average = await collection.AsQueryable().AverageAsync(x => x.Score, ct);
await collection.AsQueryable().Where(x => x.Dirty).ForEachAsync(x => x.Flush(), ct);

IBLiteQueryable<T> XML docs

Expanded <remarks> to enumerate all async terminators and explicitly warn against the inherited sync LINQ terminators on a BLite queryable.

Tests (tests/BLite.Tests/Query/BLiteQueryableExtensionsTests.cs)

42 new tests; each new method has a happy-path case and an empty-source case validating throw-vs-default semantics.

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[Copilot]

Pull request overview

Adds a comprehensive async LINQ “terminator” layer for IBLiteQueryable<T> so consumers can materialize/query results without falling back to blocking sync LINQ methods, plus updated docs and new test coverage.

Changes:

• Added async terminator extensions (e.g., FirstAsync, SingleAsync, LastAsync, ElementAtAsync, MinAsync/MaxAsync, SumAsync, AverageAsync, ForEachAsync) to BLiteQueryableExtensions.
• Expanded IBLiteQueryable<T> XML remarks to enumerate supported async terminators and warn against sync LINQ terminators on BLite queryables.
• Introduced a new dedicated test suite covering the new async terminators and their empty-sequence semantics.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 4 comments.

File	Description
src/BLite.Core/Query/BLiteQueryableExtensions.cs	Adds async materializer/terminator extensions for common LINQ end-operations over BLite queryables.
src/BLite.Core/Query/IBLiteQueryable.cs	Updates XML docs to guide users toward async terminators and away from blocking sync LINQ.
tests/BLite.Tests/Query/BLiteQueryableExtensionsTests.cs	Adds tests validating behavior (happy paths + empty-sequence behavior) for the new async extensions.

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[Copilot]

SumAsync for IAsyncEnumerable uses unchecked sum += ... for int (and similarly for the long overload). This differs from System.Linq Sum, which throws OverflowException on overflow; here it can silently wrap depending on compilation settings. Use checked arithmetic (or an explicit overflow-checked helper) to match LINQ semantics.

[Copilot]

AverageAsync for IAsyncEnumerable accumulates sum using unchecked sum += ... (and the long overload does the same). LINQ’s Average over int/long uses overflow-checked accumulation; without checked arithmetic this can silently overflow and produce incorrect averages for large sequences/values. Consider using checked addition (and keep count increment separate).

[Copilot]

Test coverage gap: the LastOrDefaultAsync(predicate) overload is only exercised for the no-match case. Add a positive predicate test that asserts it returns the last matching element to guard against regressions in the Where(...).LastOrDefaultAsync() chaining path.

[Copilot]

Test coverage gap: empty-sequence behavior is only asserted for SumAsync(int) and AverageAsync(int/decimal). Since the long/double/decimal overloads have separate implementations, add empty-source tests for SumAsync(long/double/decimal) returning 0 and AverageAsync(long/double) throwing InvalidOperationException.