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/OpenAIProvider.swift

@@ -117,7 +117,7 @@ class OpenAIProvider: AIProvider {
         }
     }
 
-    private func fallbackModels() -> [ModelInfo] {
+    func fallbackModels() -> [ModelInfo] {
         Self.knownModels.map { id, info in
             ModelInfo(
                 id: id,
@@ -283,7 +283,7 @@ class OpenAIProvider: AIProvider {
 
     // MARK: - Helpers
 
-    private func buildURLRequest(from request: ChatRequest, stream: Bool) throws -> URLRequest {
+    func buildURLRequest(from request: ChatRequest, stream: Bool) throws -> URLRequest {
         let url = URL(string: "\(baseURL)/chat/completions")!
 
         var apiMessages: [[String: Any]] = []
@@ -358,7 +358,7 @@ class OpenAIProvider: AIProvider {
         return urlRequest
     }
 
-    private func convertToChatResponse(_ apiResponse: OpenRouterChatResponse) -> ChatResponse {
+    func convertToChatResponse(_ apiResponse: OpenRouterChatResponse) -> ChatResponse {
         guard let choice = apiResponse.choices.first else {
             return ChatResponse(id: apiResponse.id, model: apiResponse.model, content: "", role: "assistant", finishReason: nil, usage: nil, created: Date())
         }