transport: return unexpected EOF for partial headers

[buger]

Fixes #9200

Track cumulative bytes read while filling a gRPC message header. If EOF occurs after any partial progress, return io.ErrUnexpectedEOF even when the final read call itself returned zero bytes.

The regression test splits a partial header and EOF across separate recv buffer messages and verifies both the returned error and flow-control byte accounting.

RELEASE NOTES:

• transport: Partial gRPC message headers followed by EOF now return io.ErrUnexpectedEOF.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.20%. Comparing base (d74861e) to head (26d88f7).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #9204      +/-   ##
==========================================
- Coverage   83.21%   83.20%   -0.01%     
==========================================
  Files         420      420              
  Lines       34008    34010       +2     
==========================================
  Hits        28299    28299              
- Misses       4275     4277       +2     
  Partials     1434     1434              

Files with missing lines	Coverage Δ
internal/transport/transport.go	92.34% <100.00%> (+1.10%)	⬆️

... and 23 files with indirect coverage changes

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