Bug: PythonUvBuilder:CopyDependencies fails when sam build --cached deletes .aws-sam/build/<FunctionName>

[tomaDev]

Description

PythonUvBuilder:CopyDependencies fails with [Errno 2] No such file or directory: '.aws-sam/build/<FunctionName>' when invoked from sam build --cached --beta-features. The cached-build path removes the build output directory but CopyDependencies expects it to already exist.

Destructive failure mode: the failed invocation also wipes .aws-sam/build/ for all functions, so users must sam build --beta-features from scratch to recover.

Filed here per @vicheey's diagnosis on aws/aws-sam-cli#8838 that the fix likely belongs in aws-lambda-builders.

Steps to reproduce

# Any SAM project with 2+ Python functions using python-uv builder
sam build --beta-features          # succeeds
sam build --cached --beta-features # fails — also wipes prior .aws-sam/build/

Observed result

Starting Build use cache
Building codeuri: /path/to/src/my_function runtime: python3.14 architecture: arm64 functions: MyFunction
Auto-detected manifest file: /path/to/src/my_function/pyproject.toml
 Running PythonUvBuilder:CleanUp
 Running PythonUvBuilder:ResolveDependencies
Building Python dependencies using UV
Found uv.lock alongside pyproject.toml - using lock-based build for precise dependencies
Building from UV lock file
 Running PythonUvBuilder:CopyDependencies

Build Failed
Error: PythonUvBuilder:CopyDependencies - [Errno 2] No such file or directory: '/path/to/.aws-sam/build/MyFunction'

.aws-sam/build.toml: every function entry has manifest_hash = "", so cache-decision logic routes through CopyDependencies against a non-existent target dir.

Expected result

sam build --cached --beta-features reuses cached deps when manifest unchanged; only re-copies source. Build dir recreated before CopyDependencies runs. No destructive state-loss on failure.

Notes

• sam build --beta-features (no --cached): works
• sam build --parallel --beta-features (no --cached): works
• sam build --cached --parallel --beta-features: same failure

Environment

Reproduced 2026-04-26:

SAM CLI: 1.158.0
aws-lambda-builders: shipped with sam-cli 1.158.0
Python: 3.14
uv: 0.11.7
OS: macOS 26.3 (aarch64-apple-darwin)

Related: aws/aws-sam-cli#8838

[tomaDev]

Opened #870 with a fix: CopyDependenciesAction had its source/destination swapped, and UvRunner.install_requirements passed a relative --target through to uv (which runs with cwd=project_dir, so deps landed under the source dir). PR fixes both, with regression coverage in test_workflow.py, test_packager.py, and the existing dependencies_dir integration test.