Map transport failures to grpc-ruby errors

[ekmartin]

Fixes #5.

Transport failures escaped ClientStub#request_response as raw Ruby exceptions (Errno::ECONNREFUSED, EOFError, OpenSSL::SSL::SSLError, …), breaking code written against grpc-ruby's contract that every failure is a GRPC::BadStatus, such as GAPIC's UNAVAILABLE retry policies.

Changes

• Connection, DNS, TLS, and HTTP/2 connection failures (including GOAWAY and refused requests) raise GRPC::Unavailable.
• HTTP/2 stream resets use gRPC's HTTP/2 status mapping: REFUSED_STREAM → Unavailable, CANCEL → Cancelled, ENHANCE_YOUR_CALM → ResourceExhausted, INADEQUATE_SECURITY → PermissionDenied, and other reported stream errors → Internal.
• The original exception is preserved as error.cause, and deferred operations record the status.
• Only the transport is covered: errors raised by marshal and unmarshal callables are unchanged.
• Requires protocol-grpc ~> 0.17, which preserves underlying body read errors (upstream fix). Transport errors propagate directly without a response-body wrapper.

TLS verification failures previously raised OpenSSL::SSL::SSLError and now raise GRPC::Unavailable, matching grpc-ruby; the TLS tests are updated accordingly.

Testing

• Each mapped error type, real refused and closed connections, a server resetting the stream after response headers, a connection failing while reading the response, and GAPIC retrying a reset connection.
• The new and updated tests fail without the fix.
• Validated against published protocol-grpc 0.17.0: 122 tests, 436 assertions, and 100% line coverage. RuboCop and public API documentation coverage pass.

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