fix(application): preserve non-success dispatcher return codes

[gcomneno]

Summary

Preserve explicit return codes from dispatcher.run() in Application._run_inner() while keeping the existing success fallback for commands that do not return a value.

Previously, _run_inner() used:

• dispatcher.run() or os.EX_OK
• dispatcher.run() or 0

This meant falsy explicit return codes could be normalized incorrectly. In particular, this change avoids relying on truthiness and only falls back when dispatcher.run() returns None.

With this change:

• None falls back to os.EX_OK
• 0 is preserved as 0
• non-zero return codes are preserved

Testing

uv run ruff check craft_application/application.py tests/unit/test_application.py tests/integration/commands/test_init.py
uv run ruff format --check craft_application/application.py tests/unit/test_application.py tests/integration/commands/test_init.py
uv run pytest -q tests/unit/test_application.py tests/integration/commands/test_init.py -rs

[gcomneno]

Hi! Just gently checking whether there’s anything I should adjust here.

This PR is intended to preserve non-success dispatcher return codes while keeping the existing success-path behaviour unchanged.

Happy to rework it if there’s a preferred direction, or to close it if this should not move forward.

[mr-cal]

LGTM, thanks! Can you resolve the conflicts so we can land this?

This class got simplified, so your changes should be even simpler (one instance of this logic instead of two).

[lengau]

LGTM, thanks!

[mr-cal]

FYI to anyone who looks at this PR in the future - this PR makes a semantic correction of doing an explicit comparison against None, rather than relying on falsiness.

But since the only falsy exit code is 0 (EX_OK) is a successful code, this doesn't make behavioral changes nor resolve canonical/snapcraft#6119.