fix(mcp): skip unreadable regions before splitting scans - #128

Why is this PR necessary, what does it do?

Fixes #127. On 64-bit Windows targets, scan_value and scan_pattern ran out of the 30 s budget before they reached the heap.

• The Windows region walk reports the whole address space, MEM_FREE and MEM_RESERVE ranges included.
• _run_batched_scan cut all of that into scan_batch_bytes pieces. At the 64 MiB default that is about 2 million batches for ~128 TiB.

This PR leaves unreadable regions out before split_regions / batch_regions:

• _run_batched_scan filters the snapshot to readable regions first. Unreadable regions can never hold a match, and every backend's scan already drops them through default_scan_filter, so the results do not change.
• find_pointer_paths applies the same filter before cutting regions for the pointer map. scan_pointer_paths already keeps only readable regions itself, so the map does not change. It just no longer builds millions of pieces first.

list_memory_regions and the session's region snapshot are untouched.

How it was tested

• Two regression tests in tests/mcp_server/test_toolset.py (TestHugeRegionsAreScannedInPieces) use a fake target with a 1 MiB unreadable region below its two readable ones.
  • Without the fix, both fail: the unreadable region turns into extra batches or pointer-map pieces.
  • With the fix, both pass.
• flake8 PyMemoryEditor tests and mypy PyMemoryEditor report no issues.
• Full suite (pytest tests) on Windows 11 x64, Python 3.13.14, with the speed extra: 1832 passed, 28 skipped, 1 failed.
  • The failure is tests/process/test_foreign_process.py, and it fails the same way on unmodified main.
  • Cause: inside a Windows venv, sys.executable is the venv launcher, which starts the real interpreter as a child, so the test opens the launcher's pid (WinError 299).
  • It passes when run with the base interpreter.
• Live check with the reproducer from MCP scan_value on 64-bit Windows targets hits the 30 s budget before reaching the heap: free/reserved address space is split into millions of pieces #127 against this branch, without --scan-batch-bytes:
  • this branch: scan_value returns the counter with count: 1, partial: false, elapsed_seconds: 0.002;
  • 3.4.1: count: 0, partial: true, elapsed_seconds: 30.122.

Checklist (complete all items):

• Added tests as necessary.
• There is no breaking change for existing features.

References:

Fixes #127

Notes:

• The change only filters on MemoryRegion.is_readable, so it is platform-independent. I could only run it on Windows; CI will cover Linux and macOS.
• Written with help from Claude (an AI assistant). I reviewed the change and ran the checks above locally.

🤖 Generated with Claude Code