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/translator/concerns/thinkingUnified.js

@@ -237,12 +237,10 @@ function applyFormat(fmt, body, cfg, caps, supportedLevels) {
     }
     case "claude-adaptive": {
       if (none && canDisable) { body.thinking = { type: "disabled" }; break; }
-      // output_config.effort alone does NOT turn thinking on: Anthropic requires
-      // an explicit thinking:{type:"adaptive"} on Opus 4.6/4.7/4.8 and Sonnet 4.6
-      // ("thinking is off unless you explicitly set it"), and Anthropic-compatible
-      // shims (e.g. GitHub Copilot /v1/messages) default thinking off even for
-      // Sonnet 5. Send both fields — the documented adaptive-thinking shape.
-      body.thinking = { type: "adaptive" };
+      // Models that can disable thinking need the explicit adaptive switch.
+      // Permanently adaptive models such as Fable 5.1 accept effort directly.
+      if (canDisable) body.thinking = { type: "adaptive" };
+      else delete body.thinking;
       const level = toLevel(eff);
       body.output_config = { effort: level === "xhigh" ? "high" : level };
       break;