Revert "Move network layer off @MainActor isolation (#21695)"

[devin-ai-integration]

Summary

Reverts PR #21695 which removed @MainActor from the network client layer. After merging to main, the app hangs immediately on launch. This revert restores the original @MainActor annotations to unblock development while the root cause is investigated.

What this restores:

• @MainActor on GatewayHTTPClient, HealthCheckClient, HostToolExecutor static methods, and ~40 network client protocols/structs
• Task { @MainActor in in BtwClient.sendMessage() (was changed to Task {)
• Synchronous resolveConnection(), isConnectionManaged(), buildURL() (were made async in the fix-up commit)
• Synchronous handleAuthenticationFailure() in GatewayConnectionManager and approvalPath() in InteractionClient

Context: The original PR also coincided with PR #21696 (SettingsStore/SigningIdentityManager changes). The hang could be from either PR or their interaction. This revert isolates #21695 to determine the cause.

Review & Testing Checklist for Human

• Build the app locally — confirm it compiles cleanly with strict concurrency checking (the revert should restore the pre-Move network layer off @MainActor isolation #21695 state exactly)
• Launch the app and verify the hang is gone — if the hang persists after this revert, the cause is PR Move blocking I/O off main thread and add signing error responses #21696 or another recent merge, not Move network layer off @MainActor isolation #21695
• Verify GatewayHTTPClient.resolveConnection() is synchronous again — the async/await changes to resolveConnection(), isConnectionManaged(), and buildURL() should all be reverted (these were in the second commit of Move network layer off @MainActor isolation #21695)

Notes

• This was a clean git revert with no merge conflicts despite intervening commits
• The @MainActor removal will be re-attempted in a future PR with more careful testing of the startup path and actor-hopping behavior, particularly the interaction between GatewayConnectionManager (MainActor) → resolveConnection() (nonisolated async) → AuthService.shared.baseURL (MainActor)

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

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[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no potential bugs to report.

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