Enforce proto limits in the descriptor by default for EDITION_2026.

The enforced limits for Edition 2026 is listed as follows. By default,
in EDITION_2026, we will allow:
- Up to 1500 fields per message.
- Up to 1000 oneofs per message.
- Up to 1200 fields per oneof.
- Up to 1700 values per enum.

Note that features.enforce_proto_limits = LEGACY_NO_EXPLICIT_LIMITS can
be used to opt out of this check.

PiperOrigin-RevId: 915452696