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/shared.js

@@ -22,6 +22,7 @@ export function mapStainlessArch() {
 
 // Anthropic API version (single source — reused across claude-format providers/executors)
 export const ANTHROPIC_API_VERSION = "2023-06-01";
+export const CLAUDE_CLI_VERSION = "2.1.258";
 
 // Shared Claude-compatible API headers (reused across claude-format providers)
 export const CLAUDE_API_HEADERS = {
@@ -34,7 +35,7 @@ export const CLAUDE_CLI_SPOOF_HEADERS = {
   "Anthropic-Version": ANTHROPIC_API_VERSION,
   "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",