Python: fix shared CFG exception-handler reachability

[yoff]

Summary

This dependent PR fixes the shared-CFG exception-handler reachability regression introduced by #21925.

• Route exceptional completion from canonical shared-CFG nodes so leaf mayThrow expressions such as bare yield and plain imports reach typed catch entries.
• Add focused inline regression coverage for both leaf cases.

The SSA phi normal-exit/module-export fix previously included here has moved into #21923 and is now inherited through the restacked #21925 base; it is intentionally no longer duplicated in this PR.

Red/green structure

1. Red: add inline MISSING expectations for bare-yield and plain-import exceptional successors.
2. Green: replace isIn with canonical injects mapping and flip those expectations green.

The red commit passes with its bug annotations; replacing each MISSING annotation with the desired result reproduces the missing-result failure. The green commit passes without bug annotations.

DCA impact

The exception fix restores catch reachability behind the removed CatchingBaseException rows in mypy and duplicated CPython results, as well as the youtube-dl UseOfExit stem. The Airflow ApiException = BaseException result and other phi-dependent flows are supplied by the inherited #21923 fix.

DCA #38556 evaluated pre-restack tree 121dd330738315cf6fc48629ab39d53667809c40. The reduced two-commit branch on the new #21925 base has the exact same Git tree, so the DCA correctness and performance conclusions remain directly applicable.

Validation

• codeql test run python/ql/test/library-tests/ControlFlow/shared-cfg-exceptions
• codeql query format --check-only python/ql/test/library-tests/ControlFlow/shared-cfg-exceptions/ExceptionReachabilityTest.ql

[Copilot]

Pull request overview

Fixes shared Python CFG/SSA regressions affecting exception-handler reachability and module-export phi uses.

Changes:

• Routes exceptional completion through canonical CFG nodes.
• Preserves synthetic normal-exit uses for generic SSA definitions.
• Adds regression tests for both fixes.

Show a summary per file

File	Description
python/ql/lib/semmle/python/controlflow/internal/AstNodeImpl.qll	Uses canonical CFG-node mapping for exception flow.
python/ql/lib/semmle/python/dataflow/new/internal/SsaImpl.qll	Supports phi definitions when resolving uses.
python/ql/test/library-tests/ControlFlow/shared-cfg-exceptions/test.py	Adds yield and import exception cases.
python/ql/test/library-tests/ControlFlow/shared-cfg-exceptions/ExceptionReachabilityTest.ql	Checks typed-handler reachability.
python/ql/test/library-tests/ControlFlow/shared-cfg-exceptions/ExceptionReachabilityTest.expected	Records expected exception results.
python/ql/test/library-tests/dataflow-new-ssa/test.py	Adds conditional module-export coverage.
python/ql/test/library-tests/dataflow-new-ssa/SsaTest.ql	Checks synthetic exit uses for phi definitions.

Review details

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• Files reviewed: 6/7 changed files
• Comments generated: 0
• Review effort level: Balanced