fix: use Python 3.8-compatible generics in models.py

[realfishsam]

Fixes #562

[realfishsam]

PR Review: VERIFIED

What This Does

Replaces three list[T] (PEP 585, Python 3.9+) annotations with List[T] (from typing) in the SubscribedAddressSnapshot dataclass in models.py, restoring Python 3.8 compatibility.

Blast Radius

• Python SDK only (sdks/python/pmxt/models.py)
• No core/TS SDK/OpenAPI impact

Findings

1. All three occurrences of lowercase list[T] in models.py are fixed (lines 483, 486, 489). Grep confirms no other instances remain.
2. List is already imported from typing on line 9 of the file -- no new import needed.
3. The rest of models.py already uses List[T] and Optional[...] consistently; these three were the only outliers.
4. An alternative approach would have been from __future__ import annotations (as PR 566 does for errors.py), but the List[T] approach is also correct and matches the existing style in this file.

PMXT Pipeline Check

• Field propagation: N/A
• OpenAPI sync: N/A
• Type safety: OK -- semantically identical annotations

Semver Impact

patch -- fixes TypeError on Python 3.8, no API change

Risk

None. typing.List[T] is equivalent to list[T] for all supported Python versions.