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).

oAITests/EmbeddingServiceTests.swift

@@ -0,0 +1,35 @@
+//
+//  EmbeddingServiceTests.swift
+//  oAITests
+//
+// SPDX-License-Identifier: PolyForm-Noncommercial-1.0.0
+// Copyright (C) 2026 Rune Olsen
+
+import Testing
+@testable import oAI
+
+@Suite("EmbeddingService serialization")
+struct EmbeddingServiceTests {
+
+    @Test("Serialize then deserialize round-trips an embedding exactly")
+    func roundTrip() {
+        let original: [Float] = [0.0, 1.0, -1.0, 3.14159, -0.0001, 1_000_000.5]
+        let data = EmbeddingService.shared.serializeEmbedding(original)
+        let restored = EmbeddingService.shared.deserializeEmbedding(data)
+        #expect(restored == original)
+    }
+
+    @Test("Serialized data is exactly 4 bytes per float")
+    func serializedSizeIsFourBytesPerFloat() {
+        let embedding: [Float] = Array(repeating: 0.5, count: 10)
+        let data = EmbeddingService.shared.serializeEmbedding(embedding)
+        #expect(data.count == 40)
+    }
+
+    @Test("Empty embedding serializes to empty data and back")
+    func emptyEmbedding() {
+        let data = EmbeddingService.shared.serializeEmbedding([])
+        #expect(data.isEmpty)
+        #expect(EmbeddingService.shared.deserializeEmbedding(data).isEmpty)
+    }
+}