ci: publish Cargo manifest changes reliably

[pathscale]

The first auto-publish implementation relied on GitHub’s head_commit.modified list, which was empty for the merged PR #48 push, so the publish job skipped endpoint-libs 3.0.2. This uses Git’s before/after SHA comparison instead: after all Rust gates pass on master, an unchanged Cargo.toml exits successfully and a changed manifest runs cargo publish directly.

This PR bumps the crate to 3.0.3 so its merge contains the manifest change needed to exercise and publish through the corrected path. Versions 3.0.1 and 3.0.2 were never published.

Validation:

• the exact parent/HEAD comparison detects this Cargo.toml change
• cargo publish --dry-run --allow-dirty --registry crates-io packages and verifies endpoint-libs 3.0.3
• actionlint passes with only the established Ubicloud runner labels excluded from its built-in label list
• workflow YAML and git diff --check pass
• no Cargo lockfile is generated or committed