Handle missing Connection header in Bedrock signing

[fallintoplace]

Problem

Bedrock signing fails before the HTTP request in some cases with:
KeyError: 'connection'

The code in src/anthropic/lib/bedrock/_auth.py removes the header with del headers["connection"]. If Connection is absent (which can happen in normal httpx default header sets), httpx.Headers.__delitem__ raises before any request is sent.

Fix

• Replace direct deletion with a case-insensitive filter of headers before signing.
• Use a new header map that omits only Connection if present:
  new_headers = {k: v for k, v in dict(headers).items() if k.lower() != "connection"}
• Pass new_headers into AWSRequest.

This mirrors the existing guard pattern used by the AWS auth path in this repo and preserves the intent of excluding Connection from signed headers without crashing on missing keys.

Scope & Behavior

• No functional behavior change when a Connection header exists.
• Safe no-op when Connection is absent.
• Prevents signing-time failures for Bedrock requests.

File Changed

• src/anthropic/lib/bedrock/_auth.py

Testing

Not run (per request).