Make NetworkingClient.headers thread-safe

[logansease]

headers was a plain stored dictionary read on every request build (and on retries from URLSession/Combine threads) while being rewritten on auth token refresh from arbitrary threads. Swift Dictionary is not thread-safe, so this raced and corrupted the CoW buffer, producing the Crashlytics crashes in NetworkingClient.addHeaders/removeHeaders and NetworkingRequest.__ivar_destroyer (EXC_BAD_ACCESS / EXC_BREAKPOINT / SIGABRT).

• back headers with an NSLock-protected private store; the public property keeps the exact same get/set surface, so existing consumers compile and behave unchanged
• add withHeaders(_:) for callers that need an atomic read-modify-write (e.g. merging auth headers) instead of racy get-modify-set
• add regression tests; the concurrent hammer test segfaults (signal 11) against the previous implementation and passes with the lock

[Copilot]

Pull request overview

This PR addresses a production crash caused by concurrent reads/writes to NetworkingClient.headers by making header storage thread-safe and adding regression tests to prevent reintroducing the data race.

Changes:

• Replaced the stored headers dictionary with a lock-protected private backing store while preserving the existing headers get/set API surface.
• Added withHeaders(_:) to support atomic read-modify-write updates for callers that need it (e.g., auth header merges).
• Added concurrency regression tests that exercise the previously-crashing access patterns.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
Sources/Networking/NetworkingClient.swift	Introduces an NSLock-protected backing store for headers and adds withHeaders(_:) for atomic mutations.
Tests/NetworkingTests/HeadersThreadSafetyTests.swift	Adds regression tests that stress concurrent reads/writes and validate atomicity/value semantics.

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