go-framework: fail early when no Go sources are present

[gcomneno]

This change makes the go-framework extension validate that the project contains at least one Go source file.

Motivation: projects with a go.mod but no .go files can otherwise reach later stages and fail with lower-level errors.

Unit tests were updated to add a minimal main.go in valid test cases.

[gcomneno]

Hi! Just gently following up on this one.

I rebased the branch onto current upstream/main and re-ran the focused checks locally:

• uv run --group dev python -m pytest -q tests/unit/extensions/test_go.py -rs
• uv run --group lint --group types ruff check rockcraft/extensions/go.py tests/unit/extensions/test_go.py
• uv run --group lint --group types ruff format --check rockcraft/extensions/go.py tests/unit/extensions/test_go.py

The focused test run passes with 16 passed, and Ruff is clean.

Happy to adjust anything else if useful.

[gcomneno]

Friendly ping on this PR. It has been idle for a while, and it should still be ready from my side.

Please let me know if you would like me to rebase, refresh the checks, or adjust anything else.

Thanks!