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/translator/thinking-unified.test.js

@@ -82,6 +82,11 @@ describe("applyThinking per provider format", () => {
     // Sonnet 5). Both fields together are the documented adaptive shape.
     expect(out.thinking).toEqual({ type: "adaptive" });
   });
+  it("Fable 5.1 → effort without a redundant thinking switch", () => {
+    const out = apply("claude", "claude-fable-5-1", { reasoning_effort: "high" }, "claude");
+    expect(out.output_config).toEqual({ effort: "high" });
+    expect(out.thinking).toBeUndefined();
+  });
   it("claude haiku → enabled+budget", () => {
     const out = apply("claude", "claude-haiku-4.5", { reasoning_effort: "high" }, "claude");
     expect(out.thinking).toEqual({ type: "enabled", budget_tokens: 24576 });