Improve error message for partial version strings

[alexchenai]

Summary

• Adds a new VersionParseErrorPartial error variant that detects when users provide partial versions like "18" or "18.4" instead of full semver versions like "18.0.0"
• The improved error message explains that Volta requires a fully-qualified semver version and suggests using volta install or volta pin on the command line, which supports semver ranges
• Adds an is_partial_version helper function with unit tests

Fixes #1433

Before

Volta error: Could not parse version "18"

Please verify the intended version.

After

Volta error: Could not parse version "18"

Volta requires a fully-qualified semver version (e.g. "18.0.0" instead of "18").

To install using a partial version, use `volta install node@18` or `volta pin node@18` on the command line, which supports semver ranges.

Test plan

• Unit tests for is_partial_version cover major-only, major.minor, full semver, and non-numeric strings
• Unit tests for VersionSpec::from_str verify partial versions still work via the semver range fallback path
• cargo test passes

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