fix: propagate anthropic-beta header and preserve cache_control.scope

[kamalkalwa]

Problem

When using targets-based configs (e.g., fallback strategies), requests to Anthropic's /v1/messages with cache_control: { type: 'ephemeral', scope: 'global' } fail with:

anthropic error: system.1.cache_control.ephemeral.scope: Extra inputs are not permitted

The same request works fine when sent directly to Anthropic. This breaks Claude Code and Agent SDK traffic through Portkey when using the prompt-caching-scope-2026-01-05 beta.

Root Cause

1. anthropic-beta header not forwarded with targets-based configs

In handlerUtils.ts, anthropicConfig (which reads anthropic-beta from request headers) is only merged into provider options for inline configs. When using targets, the header never reaches api.ts, so it falls back to the default messages-2023-12-15 beta — and Anthropic rejects the scope field as unknown.

2. cache_control.scope stripped in OpenAI-to-Anthropic transforms

Six places in chatComplete.ts hardcode cache_control: { type: 'ephemeral' }, discarding any additional fields like scope. Same issue in Bedrock's utils.ts.

Fix

• api.ts: Fall back to reading anthropic-beta and anthropic-version from the original request headers via Hono context (c.req.header(...)) when providerOptions and gatewayRequestBody don't include them
• chatComplete.ts: Preserve the original cache_control object instead of replacing it with { type: 'ephemeral' } (6 locations)
• bedrock/utils.ts: Same fix for the Bedrock Anthropic path
• Type definitions: Add optional scope field to CacheControlEphemeral and PromptCache

Files Changed

• src/providers/anthropic/api.ts — header fallback via Hono context
• src/providers/anthropic/chatComplete.ts — preserve cache_control in transforms
• src/providers/bedrock/utils.ts — preserve cache_control in Bedrock path
• src/types/MessagesRequest.ts — add scope? to CacheControlEphemeral
• src/types/requestBody.ts — add scope? to PromptCache

Fixes #1589