fix(python): guard nullable market slug cache

[realfishsam]

Summary

• Skip slug index entries when a loaded market has no slug
• Mirrors the TypeScript loadMarkets guard and prevents crashes on slugless venues

Fixes #156

Test Plan

• git diff --check
• python3 -m py_compile sdks/python/pmxt/client.py

[realfishsam]

PR Review: PASS (NOT VERIFIED)

What This Does

Guards the Python SDK market slug cache so markets with slug=None are still cached by market id but no longer create a None key in markets_by_slug.

Blast Radius

Python SDK only: Exchange.load_markets() cache population. No core/server/OpenAPI changes.

Consumer Verification

Before (base branch):
On base, a UnifiedMarket with slug=None would execute self.markets_by_slug[market.slug] = market, creating a None lookup entry. I did not run a Python consumer repro because this runner is missing pytest for the SDK test phase.

After (PR branch):
Patch adds if market.slug: before writing the slug cache. Core build passed and core Jest tests passed; full root verification stopped in the Python SDK phase with /usr/bin/python3: No module named pytest.

Test Results

• Build: PASS
• Unit tests: CORE PASS (24 suites, 644 tests; root verification then blocked by missing pytest)
• Server starts: PASS during root verification
• E2E smoke: NOT VERIFIED (Python SDK environment missing pytest)

Findings

No blocking findings.

PMXT Pipeline Check

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

Semver Impact

patch -- Python SDK cache bug fix without API shape change.

Risk

Python SDK runtime behavior was inspected from the diff but not executed end-to-end in this environment because pytest is not installed.