codegen: preserve symbol tables across copied finally bodies - #8507

Summary

• preserve the current symbol-table cursors while codegen emits the extra copy of a finally body used by an early return, break, or continue
• add coverage for lambdas, comprehensions, nested functions, and nested classes in copied finally bodies

Root cause

Leaving a try block early compiles its finally body once for the early-exit path and again for the statement's normal cleanup paths. The first copy consumed the nested-scope symbol-table cursors. When the later copy reached a comprehension or another nested scope, codegen reported SyntaxError: no symbol table available.

Saving and restoring all three cursor components around the early-exit copy makes that speculative emission cursor-neutral, matching the existing treatment of duplicated codegen paths.

This surfaced while importing pytest: _pytest/pytester.py has a generator hook that returns from a try and uses comprehensions in finally.

Validation

• cargo fmt --all -- --check
• cargo clippy --workspace --exclude rustpython_wasm --exclude rustpython-venvlauncher --exclude rustpython-capi --all-targets
• cargo test --workspace --exclude rustpython_wasm --exclude rustpython-venvlauncher --exclude rustpython-capi
• (cd crates/capi && cargo test) — 102 passed
• CI-equivalent release build with threading,jit and the explicit stdlib/sqlite/rustls feature set
• extra_tests: 410 passed
• byte-compiled pytest 9.1.1's _pytest/pytester.py

Summary by CodeRabbit

• Bug Fixes

  • Fixed try/finally handling to preserve correct behavior after returns, breaks, and continues.
  • Ensured finally blocks execute exactly once during returns.
  • Corrected generator behavior when returning from try blocks, including proper StopIteration values.
  • Preserved nested comprehension and scope behavior in duplicated finally code.

• Tests

  • Added regression coverage for control flow, generators, nested scopes, and try/finally execution.