fix(local): warn when CodeUri path does not exist before mounting

[cesar32az]

Which issue(s) does this change fix?

#1754

Why is this change necessary?

When running sam local invoke, start-api or start-lambda with a CodeUri (or layer ContentUri) that does not exist on the local machine, the path is silently mounted as an empty directory in the container. The invocation then fails with a secondary error inside the container (import/handler not found) that hides the real problem. Issue #1754 was marked open for contributors by a maintainer.

How does it address the issue?

LambdaRuntime._get_code_dir now logs a warning when the resolved code path does not exist, before the container is started:

Local code path '<path>' does not exist. It will be mounted as an empty directory in the container, which will likely cause the invocation to fail. Verify the CodeUri/ContentUri of your function or layer, or run 'sam build' if the path is a build artifact.

Since _get_code_dir is used both for function code and for layers (via _unarchived_layer), the warning covers both cases. Behavior is otherwise unchanged: the path is still returned as is, exactly as documented in the method docstring.

Verified end-to-end with Docker: sam local invoke on a template whose CodeUri points to a missing directory now prints the warning right after "Invoking app.handler (python3.12)".

What side effects does this change have?

None. Only a warning log line is added; no control flow changes. code_path values that exist (directories, zip/jar archives) behave exactly as before, and the full unit suite passes.

Mandatory Checklist

PRs will only be reviewed after checklist is complete

• Review the generative AI contribution guidelines
• Add input/output type hints to new functions/methods (no new functions; existing signature unchanged)
• Write design document if needed (not needed: single warning log line)
• Write/update unit tests (two new tests in TestLambdaRuntime_get_code_dir: warning emitted for a missing path, no warning for an existing path)
• Write/update integration tests (not needed: log-only change, covered by unit tests and manual end-to-end verification with Docker)
• Write/update functional tests if needed (not needed)
• make pr passes (schema, black-check, ruff, mypy: no issues in 1375 files; unit suite: 9287 passed, coverage 94.16% >= 94%)
• make update-reproducible-reqs if dependencies were changed (no dependency changes)
• Write documentation (not needed: no user-facing interface change)

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.