fix(go): resolve datetime aliases for marshal/unmarshal

[jsklan]

Description

Fixes Go SDK generator failing to apply custom datetime serialization when a field's type is a named alias of datetime or date (e.g. DatetimeAlias: type: datetime). Previously, alias fields fell back to default time.Time JSON handling instead of using Fern's custom internal.DateTime helpers.

Link to Devin Session
Requested by: @jsklan

Changes Made

• Alias resolution: maybeDate, maybeDateProperty, and maybeFormatStructTag now resolve named type aliases before checking for date/datetime primitives
• Test fixture: Added datetime-api fixture to validate alias handling
• Seed updates: Updated Go SDK seed outputs and added datetime-api to allowedFailures for generators that don't yet support it

Testing

• Many rounds of CI tests in other PR: fix(go,python): resolve datetime aliases; fix wiremock naming collisions #13103

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