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

open-sse/providers/registry/claude.js

@@ -1,4 +1,4 @@
-import { CLAUDE_CLI_SPOOF_HEADERS } from "../shared.js";
+import { CLAUDE_CLI_VERSION } from "../shared.js";
 
 export default {
   id: "claude",
@@ -25,7 +25,7 @@ export default {
       "Anthropic-Version": "2023-06-01",
       "Anthropic-Beta": "claude-code-20250219,oauth-2025-04-20,interleaved-thinking-2025-05-14,context-management-2025-06-27,prompt-caching-scope-2026-01-05,advanced-tool-use-2025-11-20,effort-2025-11-24,structured-outputs-2025-12-15,fast-mode-2026-02-01,redact-thinking-2026-02-12,token-efficient-tools-2026-03-28",
       "Anthropic-Dangerous-Direct-Browser-Access": "true",
-      "User-Agent": "claude-cli/2.1.258 (external, sdk-cli)",
+      "User-Agent": `claude-cli/${CLAUDE_CLI_VERSION} (external, sdk-cli)`,
       "X-App": "cli",
       "X-Stainless-Helper-Method": "stream",
       "X-Stainless-Retry-Count": "0",
@@ -58,6 +58,7 @@ export default {
   },
   models: [
     { id: "claude-opus-5", name: "Claude Opus 5" },
+    { id: "claude-fable-5-1", name: "Claude Fable 5.1" },
     { id: "claude-fable-5", name: "Claude Fable 5" },
     { id: "claude-sonnet-5", name: "Claude Sonnet 5" },
     { id: "claude-haiku-4-5-20251001", name: "Claude 4.5 Haiku" },