fix: use Python 3.8-compatible annotations in errors.py

[realfishsam]

Fixes #561

[realfishsam]

PR Review: VERIFIED

What This Does

Adds from __future__ import annotations to errors.py, making PEP 604 union syntax (str | None) and PEP 585 generics (dict[str, type[...]]) evaluate lazily at runtime, restoring Python 3.8 compatibility.

Blast Radius

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

Findings

1. The from __future__ import annotations import correctly defers evaluation of all annotations in the file, fixing str | None on lines 12, 66, 84 and dict[str, type[PmxtError]] on line 102 -- all four 3.10+ constructs are covered by this single import.
2. Consistent with existing usage in constants.py which already uses the same import.
3. No functional behavior change -- purely a compatibility fix.

PMXT Pipeline Check

• Field propagation: N/A
• OpenAPI sync: N/A
• Type safety: OK -- annotations still enforce correct types, just evaluated lazily

Semver Impact

patch -- fixes a SyntaxError on Python 3.8/3.9, no API change

Risk

None substantive. The from __future__ import annotations import is well-understood and has no side effects beyond deferred annotation evaluation.