[Thanatos9404]

Summary

Fixes #1457.

The native parser only read the whitespace before the colon of an except or except* handler when the handler had an as name. Without one, whitespace_before_colon was left at its default and the whitespace was dropped, so except A : came back as except A:. The ExceptHandler and ExceptStarHandler nodes already have the field and emit it in codegen, so the fix is only in the two Inflate impls in native/libcst/src/nodes/statement.rs: always parse whitespace_before_colon from the colon token.

A bare except : parses the same as before (whitespace_after_except=" ", whitespace_before_colon=""). The whitespace after except and before : is the same token whitespace there, and whitespace_after_except consumes it first, so it is not duplicated. There is a test for that case.

Test Plan

New tests:

• native/libcst/tests/fixtures/terrible_tries.py and starry_tries.py: whitespace before : after a name, a parenthesized tuple, an unparenthesized tuple (PEP 758) and an as name, plus a bare except :. Both parser_roundtrip.rs and libcst/tests/test_roundtrip.py round-trip these fixtures.
• libcst/_nodes/tests/test_try.py: parse and codegen cases checking that whitespace_before_colon is populated for except Exception : and except (IOError, ImportError) :, the same two for except*, and that a bare except : still parses as before.

On main plus only the new tests, the four new whitespace cases and both fixture round trips fail (except foo : comes back as except foo:); the bare except : case passes. With the fix (Ubuntu 24.04 on WSL2, Python 3.10, debug build) these pass:

• uv run poe test
• pyre check
• cargo test --manifest-path=native/Cargo.toml, with and without --no-default-features
• cargo fmt --all --manifest-path=native/Cargo.toml -- --check
• cargo clippy --manifest-path=native/Cargo.toml --all-targets --all-features (no new warnings)
• flake8 and ufmt check on test_try.py, scripts/check_copyright.py