fix type-object marshalling across the sandbox boundary

[samuelcolvin]

Modeled stdlib type objects (PosixPath, datetime., re., io.*, …) were reconstructed host-side via getattr(builtins, name), raising AttributeError since they don't live in builtins. They now resolve from their real defining module, each cached in its own PyOnceLock. The Path class maps to PurePosixPath, consistent with how Path instances round-trip.

Also fix the inverse direction: a type object passed into the sandbox (input or external-call return) previously degraded to a callable; recognized builtins and modeled stdlib types are now preserved as MontyObject::Type so they round-trip and isinstance works inside the sandbox.

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Summary by cubic

Fixes type-object marshalling across the sandbox boundary so modeled stdlib types resolve from their real modules and incoming host types are preserved by identity. pathlib.Path maps to pathlib.PurePosixPath for consistency.

• Bug Fixes
  • Sandbox → host: resolve modeled stdlib types from datetime, re, io, typing (not builtins); map pathlib.Path → pathlib.PurePosixPath; cache each resolved class in its own PyOnceLock.
  • Host → sandbox: preserve recognized builtins and modeled stdlib classes as type objects (MontyObject::Type), matching by identity; match all pathlib via issubclass(PurePath); also preserve NoneType and ellipsis; unmodeled classes still degrade to callables.
  • Added tests for type-object args/returns, identity round-trips, isinstance, and unconvertible-instance errors; updated limitations docs; exported Type from the crate root.

^{Written for commit b0d574d. Summary will update on new commits.}

[codspeed-hq]

Merging this PR will not alter performance

✅ 17 untouched benchmarks
⏩ 15 skipped benchmarks¹

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_{Comparing builtins-attrs (b0d574d) with main (7f7d08e)}

Footnotes

1. 15 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

[cubic-dev-ai]

2 issues found across 6 files

<sub>Reply with feedback, questions, or to request a fix.

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[github-actions]

Codecov Results 📊

✅ Patch coverage is 100.00%. Project has 24061 uncovered lines.
✅ Project coverage is 66.36%. Comparing base (base) to head (head).

Files with missing lines (1)

File	Patch %	Lines
crates/monty-python/src/convert.rs	100.00%	⚠️ 5 partials

Coverage diff

@@            Coverage Diff             @@
##          main       #PR       +/-##
==========================================
+ Coverage    66.09%    66.36%    +0.27%
==========================================
  Files          281       281         —
  Lines        71447     71521       +74
  Branches    153474    153638      +164
==========================================
+ Hits         47218     47460      +242
- Misses       24229     24061      -168
- Partials      4043      4067       +24

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[codecov]

Codecov Report

❌ Patch coverage is 93.58974% with 5 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
crates/monty-python/src/convert.rs	93.58%	0 Missing and 5 partials ⚠️

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