fix(tools): raise a graceful ToolError when a memory file is not valid UTF-8

[WalkingDreams798]

What

The local-filesystem memory tool reads files through _read_file_content (and its async twin _async_read_file_content), which decodes strict UTF-8 and only catches FileNotFoundError:

def _read_file_content(full_path: Path, memory_path: str) -> str:
    try:
        return full_path.read_text(encoding="utf-8")
    except FileNotFoundError as err:
        raise ToolError(...) from err

A memory file containing non-UTF-8 bytes therefore raised an uncaught UnicodeDecodeError out of view / str_replace / insert, instead of the normalized ToolError used for every other failure mode. Because the encoding is hard-coded to utf-8, this reproduces on all platforms.

Repro

import tempfile
from pathlib import Path
from anthropic.lib.tools._beta_builtin_memory_tool import BetaLocalFilesystemMemoryTool
from anthropic.types.beta import BetaMemoryTool20250818ViewCommand

tool = BetaLocalFilesystemMemoryTool(base_path=tempfile.mkdtemp())
tool.memory_root.mkdir(parents=True, exist_ok=True)
(tool.memory_root / "note.dat").write_bytes(b"\xff\xfe\x80 binary")

tool.view(BetaMemoryTool20250818ViewCommand(command="view", path="/memories/note.dat"))
# -> UnicodeDecodeError: 'utf-8' codec can't decode byte 0xff ...  (uncaught)

Change

Catch UnicodeDecodeError in both the sync and async readers and surface it as a ToolError ("… is not a valid UTF-8 text file …"). Added sync + async regression tests that write a binary file and assert the graceful ToolError.

Verification

• uv run pytest -k non_utf8 tests/lib/tools/memory_tools/test_filesystem.py → 2 passed
• uv run ruff check → all checks passed
• uv run pyright (strict) → 0 errors, 0 warnings

Note: this repo's memory-tool filesystem suite has pre-existing failures on Windows (symlink/permission-mode tests) unrelated to this change; the two added tests pass and no previously-passing test regresses.