Summary

• Default StdioServerParameters.encoding_error_handler to "replace" so a server emitting malformed UTF-8 no longer crashes the client transport.
• Malformed bytes now surface as an in-stream JSON parse error and the transport stays alive for subsequent valid messages.

Motivation

Closes #2454.

stdio_client() decoded child stdout with encoding_error_handler="strict". When a spawned server writes invalid UTF-8 bytes to stdout, the UnicodeDecodeError raised during TextReceiveStream iteration escapes the decode loop's except clauses and tears down the transport task group — it surfaces as an ExceptionGroup out of the context manager instead of being delivered as a normal in-stream parse error.

The issue itself points at the analogous server-side hardening (#2302, errors="replace" on stdin), which deliberately chose to: replace invalid bytes with U+FFFD, let JSON validation fail on the malformed line, and keep the transport alive. This change brings the client to parity: defaulting encoding_error_handler to "replace" means the bad line fails JSON-RPC validation and is delivered as an Exception via _parse_line, while later valid messages still come through.

Verification

• uv run pytest tests/client/test_stdio.py -q — 34 passed, 1 skipped
• uv run pytest tests/client/ tests/interaction/transports/test_stdio.py -q — 238 passed, 1 skipped, 1 xfailed (no regression from the default change)
• New regression test test_invalid_utf8_mid_session_surfaces_as_an_in_stream_exception fails without the fix (5s task-group hang) and passes with it.
• Manual repro from the issue (\xff\xfe\n followed by a valid ping) crashed the task group on main with an ExceptionGroup(UnicodeDecodeError, ...); with the fix it surfaces a ValidationError then reads the following valid message.
• ruff format --check + ruff check + pyright clean on both changed files.

Notes

• Salvages the intent of the stale, now-conflicting fix(stdio_client): tolerate invalid UTF-8 from child stdout #2456 by @shaun0927 (same root cause + default flip), rebuilt cleanly against current main after the transport was refactored. The decode/drain plumbing moved, so this is a fresh implementation on the new code path with a mid-session regression test (distinct from the existing test_invalid_utf8_flushed_by_a_dying_server_does_not_break_shutdown, which only covers the raw-bytes shutdown drain).