Export Python filter function type aliases from package root

[jordansilly77-stack]

Summary

• export MarketFilterFunction and EventFilterFunction from the Python package root
• keep __all__ aligned with the imported public model symbols
• add a public export regression test for the two filter function aliases

Why

Fixes #1464. The TypeScript SDK exposes these filter-function types from the package root, while the Python SDK defines and uses the same aliases internally but does not re-export them from pmxt.

Validation

• ./sdks/python/.venv/bin/python -m pytest sdks/python/tests/test_public_exports.py -q -> 9 passed
• ./sdks/python/.venv/bin/python -m pytest sdks/python/tests -q -> 259 passed, 66 deselected

[realfishsam]

PR Review: PASS (NOT VERIFIED)

What This Does

Exports Python SDK filter function type aliases (MarketFilterFunction, EventFilterFunction) from the package root and adds AST tests to keep them public.

Blast Radius

Python SDK package root and Python public-export tests only. No sidecar/core behavior changes.

Consumer Verification

Before (base branch):
pmxt.models defined the filter function aliases, but pmxt.__init__ did not import them or include them in __all__, so from pmxt import MarketFilterFunction was not a supported root import.

After (PR branch):
Static trace shows MarketFilterFunction / EventFilterFunction imported from .models and added to __all__; the new test asserts both sets. I could not execute the Python tests because this scheduled environment lacks pytest (/usr/bin/python3: No module named pytest).

Test Results

• Build: PASS for current main core build during scheduled run
• Unit tests: NOT VERIFIED for Python (pytest missing in environment); core Jest PASS on current main (699 passed, 3 skipped)
• Server starts: N/A
• E2E smoke: PASS by static import/export trace, NOT VERIFIED by runtime import

Findings

No blocking findings.

PMXT Pipeline Check

• Field propagation (3-layer): N/A
• OpenAPI sync: N/A
• Financial precision: N/A
• Type safety: OK (type aliases only)
• Auth safety: N/A

Semver Impact

patch -- restores/expands package-root type exports without changing runtime API behavior.

Risk

Runtime import was not exercised due missing pytest / Python SDK test harness dependencies in this cron environment.