fix(bedrock): don't raise KeyError when signing requests lacking a connection header

[sean-kim05]

Summary

AnthropicBedrock's SigV4 signer raises KeyError: 'connection' when the outgoing request has no Connection header.

get_auth_headers in src/anthropic/lib/bedrock/_auth.py strips the connection header before signing (it may be altered by proxies, so it must not be part of the signature) via:

headers = headers.copy()
del headers["connection"]

httpx.Headers.__delitem__ raises KeyError when the key is absent. A request can legitimately lack a Connection header:

• HTTP/2 forbids it — per RFC 9113 §8.2.2 the Connection header is not used in HTTP/2, so a client created with http2=True produces requests without it.
• Custom transports / manually-built requests may omit it.

In those cases signing crashes before the request is ever sent.

Fix

Filter the header out instead of deleting it, so an absent header is a no-op. This mirrors the sibling aws/_auth.py implementation, which already strips connection this way — now both AWS auth paths handle it identically:

new_headers = {k: v for k, v in dict(headers).items() if k.lower() != "connection"}

Testing

Added two tests in tests/lib/test_bedrock.py:

• test_get_auth_headers_without_connection_header — signing succeeds when no connection header is present (fails with KeyError before this fix).
• test_get_auth_headers_strips_connection_header — regression guard that connection is still excluded from SignedHeaders.

Verified RED (KeyError: 'connection' at _auth.py:60) → GREEN. Full test_bedrock.py + test_aws_auth.py + test_aws.py pass (113 tests); ./scripts/lint clean (ruff + pyright + mypy).