[Swift6] Fix TSan race condition in Swift6 SynchronizedDictionary

[biovolt]

Problem

SynchronizedDictionary is a struct held as a var property on shared singleton classes (URLSessionRequestBuilderConfiguration and AlamofireRequestBuilderConfiguration). When multiple concurrent network requests mutate credentialStore or managerStore from different
threads, Swift's memory exclusivity rules treat each subscript mutation as a modifying access to the entire struct — causing a real ThreadSanitizer "Swift access race".

Changes

struct → class (SynchronizedDictionary.mustache + 13 sample files)

Changing SynchronizedDictionary from a value type to a reference type makes mutations go through pointer indirection. Concurrent subscript writes now only touch the object's internal state (already protected by NSRecursiveLock), rather than triggering a modifying
access on the owning class's stored property.

var → let (URLSessionImplementations.mustache + 11 sample files, AlamofireImplementations.mustache + 2 sample files)

Since SynchronizedDictionary is now a class, the properties credentialStore and managerStore only need to hold a stable reference — they never get reassigned. Declaring them as let makes this intent explicit and prevents any residual exclusivity issues on the property
itself.
Fixes #23055

@4brunu

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Summary by cubic

Fixes a ThreadSanitizer Swift access race in Swift6 clients by making SynchronizedDictionary a class and storing stable references for credentialStore and managerStore. This removes exclusivity violations during concurrent URLSession and Alamofire requests.

• Bug Fixes
  • SynchronizedDictionary switched from struct to class so concurrent writes mutate internal state behind NSRecursiveLock rather than the owning property.
  • credentialStore and managerStore are now let on configuration singletons to keep stable references and prevent residual exclusivity issues.

^{Written for commit 3d4101a. Summary will update on new commits.}

[cubic-dev-ai]

No issues found across 29 files

[biovolt]

Sorry I have no idea why Build 21254 failed

[4brunu]

Looks good to me, CI failure is not related to this PR.