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);