refactor: remove string versioning support

[ichung08]

Description

Removes arbitrary string versioning support (VersionTypeString, NewStringVersion()) from Epoch.
Previously, NewVersion() silently fell back to creating a string-typed version for any input that
wasn't a valid date or semver, which enabled ambiguous parsing and lexicographic ordering
comparisons.

Now, NewVersion() returns an error for non-date/non-semver values, enforcing strict version
format validation at parse time. This is a breaking change.

Changes:

• Removed VersionTypeString from the VersionType enum
• Removed NewStringVersion(), WithStringVersions(), WithStrings(), and StringVersion() functions
• Removed VersionFormatString constant
• NewVersion() now returns an error instead of falling back to string for invalid formats
• Simplified isValidVersionFormat() to only accept date (YYYY-MM-DD) and semver (X.Y.Z / X.Y)
  formats
• Removed lexicographic string comparison fallback from Compare()

🎟 Issue(s)

CPP-410: Design proposal: remove string versioning, support only date + semver

🧪 Functional Testing

• All existing unit and integration tests pass after updating test expectations
• NewVersion("alpha") now returns an error with message "invalid version format 'alpha': must be
  a date (YYYY-MM-DD) or semver (X.Y.Z or X.Y)"
• NewVersion("2024-01-01") and NewVersion("1.0.0") continue to work as expected
• Middleware waterfall logic correctly rejects non-date/non-semver version strings

📸 Screenshots

📋 Checklist

• Added/updated applicable tests
• Added/updated examples in the examples/ directory
• Updated any related documentation