fix(claude): bump CC fingerprint to 2.1.258 for new-model access

Anthropic gates newly released models (e.g. claude-fable-5-1) to Claude
Code >= 2.1.251; the spoofed 2.1.92 client got HTTP 400 on every request.
Bump User-Agent + billing-header version to 2.1.258 and refresh the
providers baseline snapshot.

Co-Authored-By: Claude Code <noreply@anthropic.com>

tests/__baseline__/providers-baseline.json

@@ -92,7 +92,7 @@
       "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/2.1.92 (external, sdk-cli)",
+      "User-Agent": "claude-cli/2.1.258 (external, sdk-cli)",
       "X-App": "cli",
       "X-Stainless-Helper-Method": "stream",
       "X-Stainless-Retry-Count": "0",