Use UTF-8 when reading Bedrock test fixtures

[Ghraven]

Summary

• Read Bedrock JSON/JSONL fixture files with encoding="utf-8" instead of the platform default encoding.
• Keep the opt-in Bedrock live env-file reader deterministic across developer machines.

Problem

These test fixtures and optional live-test env files are text inputs with repo-controlled formats, but Path.read_text() defaults to the host locale. On Windows or other non-UTF-8 locales, that can make the Bedrock tests less portable than they need to be.

Before / After

Before: Bedrock fixture loading depended on the platform default text encoding.

After: the fixture and env-file readers explicitly decode as UTF-8.

Verification

• python -m py_compile tests/lib/test_bedrock_auth_conformance.py tests/lib/bedrock_live.py
• uv run --with pytest --with pytest-asyncio --with pytest-xdist --with jsonschema --with respx --with botocore python -m pytest -q tests/lib/test_bedrock_auth_conformance.py (20 passed)

[ting-hong-shieh]

Reviewed base e67afa88, head ba3c6562, and clean synthetic merge 63c91cde with current main d9029e3a.

Using the same UTF-8 env file (# café plus an ASCII assignment) in a real LC_ALL=C, UTF-8-mode-disabled subprocess:

base:               {"exception": "UnicodeDecodeError", "status": "error"}
head:               {"status": "ok", "value": "ok"}
current-main merge: {"status": "ok", "value": "ok"}

The 20 Bedrock auth conformance tests pass on both the exact head and current-main merge. Ruff lint/format, py_compile, and git diff --check also pass for the two touched files. No AWS or OpenAI API call, credentials, or external runtime service was used. I did not find a blocking issue.