[Repo Assist] perf: O(1) assembly-name dictionary lookup in convTypeRef

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Summary

Replaces the O(n) Seq.tryFind scan in convTypeRef's bestGuess step with O(1) dictionary lookups against the existing assembly-name tables.

Root Cause / Motivation

convTypeRef translates a design-time Type to its target-context equivalent (or vice-versa). For a type t that hasn't been seen before, the previous code did:

let bestGuess =
    asms |> Seq.tryFind(fun a -> a.FullName = t.Assembly.FullName)
    |> Option.bind(fun a -> tryGetTypeFromAssembly toTgt t.Assembly.FullName fullName a)

This is an O(n) scan through all target/source assemblies for every unique type. With 100–200 reference assemblies and hundreds or thousands of distinct types in a large generated schema, this can add meaningful latency during TP warm-up.

ProvidedTypesContext already maintains two name-indexed ConcurrentDictionarys:

• targetAssembliesTable_ keyed by assembly simple name (→ Choice<Assembly, exn>)
• sourceAssembliesTable_ keyed by assembly simple name (→ Assembly)

Fix

Use t.Assembly.GetName().Name as a dictionary key for O(1) look-up:

let asmSimpleName = t.Assembly.GetName().Name
let bestGuess =
    if toTgt then
        let table = getTargetAssembliesTable()
        match table.TryGetValue(asmSimpleName) with
        | true, Choice1Of2 asm -> tryGetTypeFromAssembly toTgt t.Assembly.FullName fullName asm
        | _ -> None
    else
        let table = getSourceAssembliesTable()
        match table.TryGetValue(asmSimpleName) with
        | true, asm -> tryGetTypeFromAssembly toTgt t.Assembly.FullName fullName asm
        | _ -> None

The linear scan fallback (loop) is fully preserved for the rare case where the assembly isn't in the table (e.g. type forwarding across assembly boundaries). The original getTargetAssemblies()/getSourceAssemblies() call is also preserved to populate asms for the fallback.

A new getSourceAssembliesTable() helper is added (mirrors the existing getTargetAssembliesTable()).

Trade-offs

• Simple name lookup may theoretically match the wrong assembly if two assemblies with the same simple name but different versions are present simultaneously. In practice this does not occur in type provider reference sets. The fallback loop would still catch it.
• Per-type translation is already memoised in typeTableFwd/typeTableBwd, so each unique type is resolved at most once; the benefit is most visible during initial TP warm-up with large schemas.

Test Status

✅ All 126 tests pass (dotnet test tests/FSharp.TypeProviders.SDK.Tests.fsproj -c Release)

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