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

CHANGELOG.md

@@ -40,6 +40,9 @@
 - **i18n**: pt-BR expanded to 1132 terms
 
 ## Fixes
+- **Claude Code**: add Claude Fable 5.1 and advertise Claude Code 2.1.258 in
+  both the request header and billing identity; use its permanent adaptive-thinking
+  mode with `output_config.effort`
 - **Stream**: record usage when a client closes on the terminal event — the
   Responses API has no [DONE] sentinel, so codex closed the socket on
   `response.completed` and cancelled the reader before flush() ran its usage
@@ -738,4 +741,4 @@
 # v0.4.46 (2026-05-15)
 
 ## Breaking Changes
-- Tunnel public URL changed — old tunnel links no longer work, please reconnect to get the new URL
+- Tunnel public URL changed — old tunnel links no longer work, please reconnect to get the new URL