fix(copilot): use explicit utf-8 for ACP file shim

[xiao-wu-z]

What does this PR do?

Uses explicit UTF-8 encoding for the Copilot ACP fs/read_text_file and fs/write_text_file shim so non-ASCII text is handled consistently even when the platform default encoding is not UTF-8.

Fixes #38119

Type of Change

• 🐛 Bug fix (non-breaking change that fixes an issue)
• ✅ Tests (adding or improving test coverage)

Changes Made

• Updated agent/copilot_acp_client.py to pass encoding="utf-8" for ACP text file reads/writes.
• Added a regression test in tests/agent/test_copilot_acp_client.py that simulates a cp1252 default encoding and verifies non-ASCII content is written/read as UTF-8.

How to Test

1. scripts/run_tests.sh tests/agent/test_copilot_acp_client.py
2. .venv/bin/ruff check agent/copilot_acp_client.py tests/agent/test_copilot_acp_client.py

Verification

• scripts/run_tests.sh tests/agent/test_copilot_acp_client.py: 8/8 passed
• .venv/bin/ruff check agent/copilot_acp_client.py tests/agent/test_copilot_acp_client.py: All checks passed
• git diff --check: no whitespace errors

[teknium1]

Thanks for the focused regression fix. The premise remains valid on current main: agent/copilot_acp_client.py:706 still calls path.read_text() without an encoding for fs/read_text_file, and agent/copilot_acp_client.py:735 does the same for path.write_text(...) in fs/write_text_file.

The PR commit 020aeef5873d3b3817524888324adcdf182c93e3 changes both exact handlers to explicit UTF-8 and its added test drives both ACP JSON-RPC methods through the existing dispatch helper under a simulated cp1252 default. A repository-wide search found these ACP file-method strings only in this shim and its focused test, so there is no unaddressed sibling ACP handler.

This is an automated hermes-sweeper review.

[teknium1]

Closing — the ACP shim site is already explicit-utf-8 on current main (PLW1514 gate). Thanks.