fix(client/stdio): default encoding_error_handler to 'replace' for resilient UTF-8 decoding

[sarvesh1327]

Summary

• Changes the default encoding_error_handler in StdioServerParameters from "strict" to "replace".
• Malformed UTF-8 bytes from a child server stdout previously crashed the client transport with UnicodeDecodeError.
• With "replace", invalid bytes become U+FFFD and the line fails JSON-RPC validation, which is surfaced as an in-stream Exception that the session can handle.
• This mirrors the server-side stdio hardening from PR fix: handle non-UTF-8 bytes in stdio server stdin #2302.

Test Plan

• Reproduced the crash with a child process emitting \xff\xfe\n followed by valid JSON-RPC.
• Verified the fix: first item is a ValidationError, second item is the valid SessionMessage.
• Added regression test test_invalid_utf8_from_the_server_surfaces_as_an_in_stream_exception.
• Full tests/client/test_stdio.py suite passes (34 passed, 1 skipped).

Notes

• Fixes stdio_client crashes on malformed UTF-8 from child stdout instead of surfacing parse error #2454
• Scope intentionally small.