Preserve underlying body read errors

[samuel-williams-shopify]

A failed HTTP body can report empty? while its next read raises the pending error. The gRPC reader skipped that read, hiding failures as EOF or replacing them with misleading truncated-frame errors. Read until nil instead and propagate the original exception unchanged, while preserving clean EOF and truncated-frame handling.

Add regression coverage for failed empty bodies, partial prefixes, missing/partial payloads, and failures while finishing after a complete message. All five error regressions fail before the fix.

This supplies the shared reader fix needed to remove the ResponseBody workaround in async-grpc-compatible#9.

Validation on Ruby 4.0.7:

• Protocol suite: 233 tests, 464 assertions; 100% line coverage.
• External async-grpc suite: 84 tests, 201 assertions.
• async-grpc-compatible PR Move header helpers to metadata #9 with this reader and its ResponseBody workaround removed: 122 tests, 436 assertions.
• RuboCop and documentation coverage pass.