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/Services/EmbeddingService.swift

@@ -348,7 +348,7 @@ final class EmbeddingService {
     // MARK: - Serialization
 
     /// Serialize embedding to binary data (4 bytes per float, little-endian)
-    func serializeEmbedding(_ embedding: [Float]) -> Data {
+    nonisolated func serializeEmbedding(_ embedding: [Float]) -> Data {
         var data = Data(capacity: embedding.count * 4)
         for value in embedding {
             var littleEndian = value.bitPattern.littleEndian
@@ -360,7 +360,7 @@ final class EmbeddingService {
     }
 
     /// Deserialize embedding from binary data
-    func deserializeEmbedding(_ data: Data) -> [Float] {
+    nonisolated func deserializeEmbedding(_ data: Data) -> [Float] {
         var embedding: [Float] = []
         embedding.reserveCapacity(data.count / 4)