Phase 2 seam: expose pure helpers for testing (no logic changes)

Every change here is either dropping `private` (still invisible
outside the module, @testable import just needs internal-or-wider)
or converting a self-independent instance method to `static func`
(ChatViewModel.inferProvider/calculateCost/detectGoodbyePhrase --
none of the three ever touched `self`, and constructing a real
ChatViewModel triggers a real network call in init, so static-ifying
them sidesteps that entirely rather than fighting it). Call sites
updated to `Self.foo(...)` where the static conversion required it.

Touches: GitSyncService's URL/secret-scanning helpers,
EmbeddingService's embedding (de)serialization, ContextSelectionService's
importance scoring, and the request-building/response-parsing helpers
on all four providers (OpenRouter, Ollama, OpenAI, Anthropic) -- the
core AI request/response layer, previously 100% untested.

Verified: full existing test suite (27 tests) still green, no
regressions. Tests for these functions land in the next commit.
Phase 2 of the test-suite rollout plan (peaceful-baking-kurzweil).

oAI/Providers/OllamaProvider.swift

@@ -275,7 +275,7 @@ class OllamaProvider: AIProvider {
 
     // MARK: - Helpers
 
-    private func buildRequestBody(from request: ChatRequest, stream: Bool) -> [String: Any] {
+    func buildRequestBody(from request: ChatRequest, stream: Bool) -> [String: Any] {
         var messages: [[String: Any]] = []
 
         // Add system prompt as a system message
@@ -301,7 +301,7 @@ class OllamaProvider: AIProvider {
         return body
     }
 
-    private func parseOllamaResponse(_ json: [String: Any], model: String) -> ChatResponse {
+    func parseOllamaResponse(_ json: [String: Any], model: String) -> ChatResponse {
         let message = json["message"] as? [String: Any]
         let content = message?["content"] as? String ?? ""
         let promptTokens = json["prompt_eval_count"] as? Int ?? 0