fix(claude): support Fable 5.1

- add claude-fable-5-1 to the Claude Code model catalog (1M context,
  permanent adaptive thinking)
- centralize the spoofed Claude Code version and update both request
  and billing identities to 2.1.257 (Fable 5.1 rejects < 2.1.251)
- send output_config.effort without the redundant thinking switch for
  permanently adaptive models
- add regression coverage for capabilities, headers, billing identity
  and adaptive-effort payload

# Conflicts:
#	open-sse/providers/registry/claude.js
#	open-sse/providers/shared.js
#	open-sse/utils/claudeCloaking.js
#	tests/__baseline__/providers-baseline.json

tests/unit/capabilities.test.js

@@ -32,6 +32,13 @@ describe("getCapabilitiesForModel", () => {
     }
   });
 
+  it("reports Claude Fable 5.1 as a permanent adaptive-thinking model", () => {
+    expect(getCapabilitiesForModel("claude", "claude-fable-5-1")).toMatchObject({
+      ...claudeSonnet5Expected,
+      thinkingCanDisable: false,
+    });
+  });
+
   it("reports Kiro Claude Opus 4.8 as a 1M context model", () => {
     expect(getCapabilitiesForModel("kiro", "claude-opus-4.8").contextWindow).toBe(1000000);
     expect(getCapabilitiesForModel("kiro", "anthropic/claude-opus-4.8").contextWindow).toBe(1000000);

tests/unit/claude-cloaking.test.js

@@ -7,9 +7,14 @@
  */
 
 import { describe, it, expect } from "vitest";
-import { cloakClaudeTools, decloakStreamChunk } from "../../open-sse/utils/claudeCloaking.js";
+import { applyCloaking, cloakClaudeTools, decloakStreamChunk } from "../../open-sse/utils/claudeCloaking.js";
 import { CLAUDE_TOOL_SUFFIX } from "../../open-sse/config/appConstants.js";
 
+it("advertises a Claude Code version accepted by Fable 5.1", () => {
+  const body = applyCloaking({ messages: [] }, "sk-ant-oat-test", "session-id");
+  expect(body.system[0].text).toMatch(/^x-anthropic-billing-header: cc_version=2.1.258\./);
+});
+
 describe("cloakClaudeTools", () => {
   const baseBody = {
     tools: [{ name: "todo_write", description: "write todos", input_schema: { type: "object", properties: {} } }],

tests/unit/claude-header-forwarding.test.js

@@ -29,6 +29,7 @@ describe("DefaultExecutor.buildHeaders() — claude provider", () => {
       headers["Anthropic-Version"] === "2023-06-01" ||
       headers["anthropic-version"] === "2023-06-01";
     expect(hasVersion).toBe(true);
+    expect(headers["User-Agent"]).toBe("claude-cli/2.1.258 (external, sdk-cli)");
   });
 
   it("includes heavy-agent beta flags for claude-opus-5", () => {