refactor(models): centralize account-trade timestamp parsing

[naruto11eth]

Removes the duplicated timestamp parser in models/clob/account.py in favor of the shared validators in models/clob/_validators.py (DEV-205).

• Deletes the local _parse_epoch / _parse_optional_epoch / _EPOCH_MS_THRESHOLD; the five datetime fields (created_at, expires_at, matched_at, updated_at, timestamp) now use the shared RequiredEpochOrIsoTimestamp / EpochOrIsoTimestamp annotated types.
• Moves the "naive ISO string → UTC" rule into the shared _parse_epoch_or_iso_timestamp so account behavior is preserved (it previously did replace(tzinfo=UTC) locally).

Behavior is preserved for all realistic inputs (epoch s/ms as int or string, ISO with Z, naive ISO, offset ISO). Two intentional refinements on inputs that can't legitimately occur for these fields:

• Negative-string epochs (e.g. "-1") are now rejected (the shared parser accepts only unsigned digit strings) — trade/order timestamps are never negative; locked in by a new test.
• BuilderTrade (the other consumer of the shared parser) now coerces a naive ISO timestamp to UTC instead of leaving it naive — consistent, tz-aware datetimes; its tests use Z-suffixed inputs so they're unaffected.

Full unit suite green; ruff + pyright strict clean.

Refs DEV-205

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Note

Low Risk
Refactor of model validation with behavior preserved for normal API inputs; minor tightening on invalid edge-case strings and naive ISO handling for other shared-parser consumers.

Overview
Removes duplicate epoch/ISO timestamp parsing from account.py and wires OpenOrder, ClobTrade, and Notification datetime fields through shared RequiredEpochOrIsoTimestamp / EpochOrIsoTimestamp types from _validators.py.

The shared _parse_epoch_or_iso_timestamp now assigns UTC to naive ISO strings (previously done only in account), so all consumers get timezone-aware datetimes. Negative numeric strings (e.g. "-1") are no longer accepted as epochs—only unsigned digit strings—matching realistic API timestamps; covered by a new ClobTrade test.

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