Phase 2: tests for the provider layer and other exposed pure helpers

66 new tests across 8 files (93 total in the suite now):

- GitSyncServiceTests: convertToSSH/injectCredentials/sanitizeFilename/
  detectSecretsInText, including a below-threshold false-positive check
  on the secret regex.
- ChatViewModelPureLogicTests: detectGoodbyePhrase, inferProvider,
  calculateCost -- including the cache-read (0.1x) / cache-write (1.25x)
  pricing multipliers, which is real billing-affecting logic.
- EmbeddingServiceTests / ContextSelectionServiceTests: embedding
  (de)serialization round-trip, importance-score weighting, and the
  token-estimate fallback (content.count / 4) when no real count exists.
- OpenRouterProviderTests / OllamaProviderTests / OpenAIProviderTests /
  AnthropicProviderTests: request-building (attachments, online mode,
  cache_control breakpoints, o1/o3 temperature omission, tool schema
  conversion) and response-parsing (text, tool_use blocks, empty-choices
  fallback behavior) for all four providers, with no network involved.

Also marks calculateCost/inferProvider/detectGoodbyePhrase (ChatViewModel)
and serializeEmbedding/deserializeEmbedding (EmbeddingService) as
`nonisolated` -- discovered via the actual test failures, not
speculation: the project's `SWIFT_DEFAULT_ACTOR_ISOLATION = MainActor`
setting isolates the classes that carry an explicit `@MainActor`
(ChatViewModel), so calling their static members from a plain
synchronous @Test needs the pure ones marked `nonisolated`. Matches
the existing convention already used elsewhere in EmbeddingService
(cosineSimilarity was already nonisolated before this change).

Phase 2 of the test-suite rollout plan (peaceful-baking-kurzweil).

oAI/ViewModels/ChatViewModel.swift

@@ -421,7 +421,7 @@ Don't narrate future actions ("Let me...") - just use the tools.
 
     func inferProviderPublic(from modelId: String) -> Settings.Provider? { Self.inferProvider(from: modelId) }
 
-    static func inferProvider(from modelId: String) -> Settings.Provider? {
+    nonisolated static func inferProvider(from modelId: String) -> Settings.Provider? {
         // OpenRouter models always contain a "/" (e.g. "anthropic/claude-3-5-sonnet")
         if modelId.contains("/") { return .openrouter }
         // Anthropic direct (e.g. "claude-sonnet-4-5-20250929")
@@ -2038,7 +2038,7 @@ Don't narrate future actions ("Let me...") - just use the tools.
     }
 
     /// Detect goodbye phrases in user message
-    static func detectGoodbyePhrase(in text: String) -> Bool {
+    nonisolated static func detectGoodbyePhrase(in text: String) -> Bool {
         let lowercased = text.lowercased()
         let goodbyePhrases = [
             "bye", "goodbye", "bye bye", "good bye",
@@ -2259,7 +2259,7 @@ Don't narrate future actions ("Let me...") - just use the tools.
     /// pricing when present: cache writes cost 1.25x the base input rate, cache
     /// reads cost 0.1x. `usage.promptTokens` is already the uncached remainder —
     /// it does not need cache tokens subtracted from it.
-    static func calculateCost(usage: ChatResponse.Usage, pricing: ModelInfo.Pricing) -> Double {
+    nonisolated static func calculateCost(usage: ChatResponse.Usage, pricing: ModelInfo.Pricing) -> Double {
         let inputCost = Double(usage.promptTokens) * pricing.prompt / 1_000_000
         let cacheReadCost = Double(usage.cacheReadInputTokens ?? 0) * pricing.prompt * 0.1 / 1_000_000
         let cacheWriteCost = Double(usage.cacheCreationInputTokens ?? 0) * pricing.prompt * 1.25 / 1_000_000