Remove residual @MainActor overhead from nonisolated network layer

[devin-ai-integration]

Summary

After #21729 moved the network layer off @MainActor, two residual isolation issues remained:

1. Unnecessary main-thread hop in BtwClient.sendMessage()
The Task { @MainActor in ... } wrapper forces every BTW streaming request onto the main thread, even though BtwClient and all downstream methods (streamBtw(), GatewayHTTPClient.streamPostWithRetry()) are now nonisolated. Changed to Task { ... } so the work runs on the cooperative thread pool.

Why this matters: Every BTW side-chain message unnecessarily blocked the main thread for the duration of SSE stream setup. With the network layer nonisolated, this hop has zero purpose and adds latency + main-thread contention.

2. Swift 6 cross-isolation errors in AuthService.resolveBaseURL()
The nonisolated functions resolveBaseURL() and normalizedBaseURL() reference platformURLOverrideEnvironmentKey, authServiceBaseURLDefaultsName, and defaultBaseURL — all private static let on the @MainActor class. Swift 6 language mode treats this as a cross-isolation error because static properties on a @MainActor class inherit actor isolation, even if they are immutable.

Fix: Move the three constants to module-level private let declarations (nonisolated by default). The values are unchanged — only their declaration site moved.

Why this is safe:

• BtwClient: streamBtw() only calls GatewayHTTPClient.streamPostWithRetry() (nonisolated), JSONSerialization (thread-safe), and AsyncThrowingStream.Continuation (Sendable). No MainActor-dependent code.
• AuthService: All three constants are immutable let values initialized once. Moving them to module scope changes when they initialize (module load vs. class static init) but this is equivalent — the values are compile-time #if DEBUG branches with no runtime dependencies.

References:

• SE-0316 — Global actors — static properties on @MainActor types inherit isolation
• Migrating to Swift 6 — Apple — resolving cross-isolation access errors
• WWDC25 — Embracing Swift concurrency — @MainActor for UI state only

Review & Testing Checklist for Human

• Build in Xcode with Swift 6 language mode — verify the three cross-isolation warnings (platformURLOverrideEnvironmentKey, authServiceBaseURLDefaultsName, defaultBaseURL) are gone and no new warnings appear.
• Verify BTW side-chain streaming works — send a BTW message and confirm text deltas stream back correctly. This is the only code path affected by the Task change.
• Verify base URL resolution — confirm the app connects to the correct backend. Test that VELLUM_PLATFORM_URL environment override and debug UserDefaults fallback still resolve correctly.

Notes

• 2 files changed, no logic changes — only where constants are declared and which Task annotation is used.
• All CI checks skipped (macOS/Xcode project, no macOS runners). Local Xcode build is the primary validation gate.

Link to Devin session: https://app.devin.ai/sessions/e19a9c46f61546368efaba6020397e17
Requested by: @ashleeradka

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[devin-ai-integration]

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