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
Validation-only refactor in API response models; behavior changes are limited to invalid or edge-case inputs and are covered by existing and new unit tests.

Overview
Removes duplicated epoch/ISO timestamp parsing from models/clob/account.py and wires account CLOB models to the shared RequiredEpochOrIsoTimestamp / EpochOrIsoTimestamp validators in _validators.py.

Shared parser: _parse_epoch_or_iso_timestamp now treats naive ISO strings as UTC (aligned with epoch parsing and the old account-local behavior). Other consumers of the same helper (e.g. builder trade models) get UTC-aware datetimes for naive ISO inputs.

Account models: OpenOrder, ClobTrade, and Notification drop per-field @field_validator hooks; timestamp fields are declared with the annotated types only.

Stricter edge case: unsigned digit strings only for string epochs—values like "-1" are rejected; covered by a new unit test.

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[cesarenaldi]

how does the type on this model looks like? As we aim at normalizing it should appear as datetime.