* refactor(schemas): share identifier and canonical base64 patterns
* test: hold the offset datetime schema to the validator it replaces
The module documents that its accepted set matches z.string().datetime({ offset: true }), and nothing in the suite checked that claim. Adds table-driven accept and reject cases (leap years including the 1900/2000 century pair, month, day, hour and offset ranges, fractional and omitted seconds, missing offset, basic format) plus a differential assertion that classifies every case exactly as the replaced validator does, and asserts the emitted pattern compiles under u and v while the replaced pattern does not compile under v.
---------
Co-authored-by: felipeatp <felipe.troncosopt@gmail.com>
