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/AnthropicProvider.swift

@@ -571,7 +571,7 @@ class AnthropicProvider: AIProvider {
 
     // MARK: - Request Building
 
-    private func buildURLRequest(from request: ChatRequest, stream: Bool) throws -> (URLRequest, Data) {
+    func buildURLRequest(from request: ChatRequest, stream: Bool) throws -> (URLRequest, Data) {
         let url = messagesURL
 
         // Separate system message
@@ -685,7 +685,7 @@ class AnthropicProvider: AIProvider {
         return (urlRequest, bodyData)
     }
 
-    private func parseResponse(data: Data) throws -> ChatResponse {
+    func parseResponse(data: Data) throws -> ChatResponse {
         guard let json = try? JSONSerialization.jsonObject(with: data) as? [String: Any] else {
             throw ProviderError.invalidResponse
         }
@@ -741,7 +741,7 @@ class AnthropicProvider: AIProvider {
         )
     }
 
-    private func convertParametersToDict(_ params: Tool.Function.Parameters) -> [String: Any] {
+    func convertParametersToDict(_ params: Tool.Function.Parameters) -> [String: Any] {
         var props: [String: Any] = [:]
         for (key, prop) in params.properties {
             var propDict: [String: Any] = [