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/Services/ContextSelectionService.swift

@@ -214,7 +214,7 @@ final class ContextSelectionService {
     // MARK: - Importance Scoring
 
     /// Calculate importance score (0.0 - 1.0) for a message
-    private func getImportanceScore(_ message: Message) -> Double {
+    func getImportanceScore(_ message: Message) -> Double {
         var score = 0.0
 
         // Factor 1: Cost (expensive calls are important)
@@ -248,7 +248,7 @@ final class ContextSelectionService {
     // MARK: - Token Estimation
 
     /// Estimate token count for messages (rough approximation)
-    private func estimateTokens(_ messages: [Message]) -> Int {
+    func estimateTokens(_ messages: [Message]) -> Int {
         var total = 0
         for message in messages {
             if let tokens = message.tokens {