fix(copilot_acp): read/write ACP files as UTF-8

[solyanviktor-star]

Problem

The ACP JSON-RPC client's file handlers in agent/copilot_acp_client.py read and write with no explicit encoding:

# fs/read_text_file
content = path.read_text()                              # L706
# fs/write_text_file
path.write_text(str(params.get("content") or ""))       # L735

This is the file-I/O surface the ACP peer uses to read and edit real project files — content is arbitrary text (source code, markdown, any language). read_text()/write_text() with no encoding fall back to the system locale (cp1252/GBK on Windows). On a non-UTF-8 host:

• writing a file whose content has any non-ASCII char (unicode identifiers, comments, emoji, CJK) raises UnicodeEncodeError;
• reading such a file raises UnicodeDecodeError.

Both are caught by the handler's except Exception and returned to the peer as a JSON-RPC -32602 error — so the agent simply fails to read or edit non-ASCII files on Windows. The codebase treats all text files as UTF-8 (see hermes_cli/doctor.py / config.py), and the tool-side file I/O in tools/ already pins encoding="utf-8"; these two ACP handlers diverged.

Repro (Windows, cp1251)

p.write_text("完了 ✅")                       # UnicodeEncodeError: 'charmap' codec can't encode
p.write_bytes("完了".encode("utf-8")); p.read_text()   # UnicodeDecodeError
p.write_text("完了 ✅", encoding="utf-8")     # ok

Fix

Pin encoding="utf-8" on both calls.

Tests

Added round-trip regression tests to tests/agent/test_copilot_acp_client.py:

• test_write_text_file_writes_utf8 — Cyrillic/CJK/emoji content persists as UTF-8;
• test_read_text_file_reads_utf8 — a UTF-8 file with non-ASCII content decodes back intact.

Both fail on the pre-fix handlers and pass after. python -m pytest tests/agent/test_copilot_acp_client.py — 13 passed.

🤖 Generated with Claude Code

[alt-glitch]

This was generated by AI during triage.

Duplicate of #38121 (earlier open fix, 2026-06-03) — both pin encoding="utf-8" on the same two ACP fs handlers (fs/read_text_file / fs/write_text_file) in agent/copilot_acp_client.py with the same regression tests. Canonical is the earlier #38121; both address issue #38119.

[solyanviktor-star]

Thanks for the triage — you're right, #38121 (opened 2026-06-03) pins encoding="utf-8" on the same two ACP fs handlers with the same regression tests and predates this by a month. Deferring to it and closing this in favor of the canonical #38121 to keep the queue clean.