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/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", () => {