Preserve transport ConnectErrors in client calls

[fallintoplace]

Summary

• preserve transport-produced ConnectErrors instead of remapping them to Unavailable
• apply the shared transport error mapper across unary, unary GET, server-streaming, client-streaming, and bidi client calls
• add regressions covering the real plaintext-client-to-https:// configuration mismatch across all five call shapes

Why

HttpClient::send() already classifies local scheme/config mismatches as InvalidArgument, but the higher-level client call helpers wrapped every transport error as Unavailable. That made local caller bugs look like retryable outages and obscured the real cause.

Impact

• local transport/configuration mistakes now retain their original ConnectError classification
• generic transport failures still map to Unavailable
• wrapped transport errors that carry a ConnectError in their source chain are preserved too

Validation

• cargo test -p connectrpc --features client preserves_transport_connect_error
• cargo test -p connectrpc transport_send_error_mapper_preserves_connect_error_in_source_chain
• cargo test -p connectrpc --features client http_client_plaintext_rejects_https

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