feat(mcp/v2): wire aws_sigv4 through the egress override

Map aws_sigv4 in _to_server_spec (reusing _to_aws_sigv4_config), so MCPServerManagerV2's override
resolves it to the botocore SigV4 signer (an httpx.Auth) and lists tools through UpstreamConnection
instead of the old aws_auth seam. AssumeRole-with-explicit-base-keys still defers to v1.

Live-validated: the aws_sigv4 mode through the override against the harness (:9200) lists echo with
an AWS4-HMAC-SHA256-signed request.

litellm/proxy/_experimental/mcp_server/v2_resolver_bridge.py

@@ -146,6 +146,15 @@ def to_server_spec(server: MCPServer) -> Optional[ServerSpec]:
                 key_source=SharedKey(value=SecretStr(token)),
             ),
         )
+    if server.auth_type == MCPAuth.aws_sigv4:
+        # Reuse the SigV4 config builder; the resolver's aws_sigv4 arm turns it into the botocore
+        # signer (an httpx.Auth) that signs each upstream request.
+        aws_config = _to_aws_sigv4_config(server)
+        if aws_config is None:
+            return None  # AssumeRole with explicit base keys: not representable yet, defer to v1
+        return ServerSpec(
+            server_id=server.server_id, resource=resource, config=aws_config
+        )
     if server.has_token_exchange_config:
         # token_exchange takes precedence over client_credentials (matches v1's cascade); the arm
         # binds the exchanged token to this resource (audience, RFC 8707).

tests/test_litellm/proxy/_experimental/mcp_server/test_v2_resolver_bridge.py

@@ -413,3 +413,28 @@ async def test_none_without_passthrough_maps_to_none():
     spec = to_server_spec(server)
     assert spec is not None
     assert isinstance(spec.config, NoneConfig)
+
+
+async def test_aws_sigv4_server_maps_to_config():
+    from litellm.proxy._experimental.mcp_server.v2_resolver_bridge import to_server_spec
+    from litellm.proxy.gateway.mcp.outbound_credentials.types import (
+        AwsSigV4Config,
+        StaticKeys,
+    )
+
+    server = MCPServer(
+        server_id="sig1",
+        name="sig1",
+        transport=MCPTransport.http,
+        url="https://up.example/mcp",
+        auth_type=MCPAuth.aws_sigv4,
+        aws_access_key_id="AKIA",
+        aws_secret_access_key="secret",
+        aws_region_name="us-west-2",
+        aws_service_name="bedrock-agentcore",
+    )
+    spec = to_server_spec(server)
+    assert spec is not None
+    assert isinstance(spec.config, AwsSigV4Config)
+    assert isinstance(spec.config.credentials, StaticKeys)
+    assert spec.config.region == "us-west-2"