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/EmbeddingService.swift

@@ -348,7 +348,7 @@ final class EmbeddingService {
     // MARK: - Serialization
 
     /// Serialize embedding to binary data (4 bytes per float, little-endian)
-    private func serializeEmbedding(_ embedding: [Float]) -> Data {
+    func serializeEmbedding(_ embedding: [Float]) -> Data {
         var data = Data(capacity: embedding.count * 4)
         for value in embedding {
             var littleEndian = value.bitPattern.littleEndian
@@ -360,7 +360,7 @@ final class EmbeddingService {
     }
 
     /// Deserialize embedding from binary data
-    private func deserializeEmbedding(_ data: Data) -> [Float] {
+    func deserializeEmbedding(_ data: Data) -> [Float] {
         var embedding: [Float] = []
         embedding.reserveCapacity(data.count / 4)