Fix #4586: parser cache collides on closed generics

[jeremydmiller]

Closes #4586.

Bug

LinqParsing.FindMethodParser cached the result of resolving a custom IMethodCallParser by (Module, MetadataToken). For generic methods like Enumerable.Contains<T>, the MetadataToken is the same regardless of the closed-over T — the token points to the generic definition in the metadata table, not the reified closed method.

Consequence: the first parser that matched any T was returned for every subsequent T on the same generic method, even when the new T didn't satisfy the parser's Matches() guard. From the issue's repro:

options.Linq.MethodCallParsers.Insert(0, new StrongIdContainsParser());
// Matches only Enumerable.Contains<StrongId>

await session.Query<Doc>()
    .Where(doc => Enumerable.Contains(strongIds, doc.ExternalId))
    .ToListAsync();
// ^ Hits StrongIdContainsParser as designed.

await session.Query<Doc>()
    .Where(doc => Enumerable.Contains(names, doc.Name))
    .ToListAsync();
// ^ BUG: also hits StrongIdContainsParser because the cache slot for
//   (Enumerable module, Contains<T> metadata token) was already filled.

Fix

Key the cache by MethodInfo. MethodInfo.Equals / GetHashCode on the closed reified method include the generic arguments, so each closed generic gets its own cache slot:

-private ImHashMap<Module, ImHashMap<int, IMethodCallParser>> _methodParsersByModule = ...;
+private ImHashMap<MethodInfo, IMethodCallParser> _methodParsers = ...;

FindMethodParser simplifies to a single map lookup + AddOrUpdate. Drops the two-level (Module → int → parser) map from #4374 — that nesting only existed to keep the inner int slot small, and the int slot was the collision source.

Test

LinqTests/Bugs/Bug_4586_parser_cache_collides_on_closed_generics.cs mirrors the issue's repro:

1. Insert a custom Enumerable.Contains parser gated on T == Bug4586StrongId at position 0.
2. Run a query that should hit it — assert a controlled throw + HitCount == 1.
3. Run a second query with Enumerable.Contains<string> — must NOT hit the custom parser; assert HitCount stays at 1 + the query round-trips normally.

Regression check

custom_linq_extensions.query_with_custom_parser (the existing custom-parser path) + 44 other Contains / IsOneOf / IsNotOneOf tests still pass locally.

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