fix: handle non-ASCII file paths in build graph (closes #24)

[cortex-assistant]

Summary

Fixes #24.

The build graph serialization in samcli/lib/build/build_graph.py uses Path.read_text() and Path.write_text() without specifying encoding='utf-8'. On systems where the locale defaults to ASCII (e.g., LANG=C in Docker containers or CI), this causes a UnicodeDecodeError when the codeuri contains non-ASCII characters (e.g., src/données).

Changes

• Added encoding='utf-8' to all read_text() and write_text() calls in build_graph.py
• Added regression tests with non-ASCII codeuri paths

Test plan

• Added unit tests verifying build graph read/write with non-ASCII codeuri values
• All existing tests continue to pass

Closes #24