fix(python-sdk): export feed client support types

[realfishsam]

Summary

• Export Python FeedClient support types at the package root: Ticker, Tickers, OHLCV, Market, FeedMarket, and OracleRound.
• Add a public-export regression test for the feed support types.

Fixes #1170

Test Plan

• python3 -m py_compile sdks/python/pmxt/feed_client.py sdks/python/pmxt/__init__.py sdks/python/tests/test_public_exports.py
• AST regression check that the feed support types are imported from feed_client and present in __all__
• git diff --check

Notes

• python3 -m pytest was not available in this cron image (No module named pip / no pytest command), so I ran the equivalent targeted AST assertions directly.
• npm run generate:docs --workspace=pmxt-core produced broad unrelated API reference/config drift, so those generated files were reverted to keep this PR scoped.

[realfishsam]

CI is currently red on generated-sync checks that are unrelated to this focused export fix:

• Verify COMPLIANCE.md is up-to-date
• Verify client.py methods are up-to-date
• Verify client.ts methods are up-to-date
• Verify API_REFERENCE.md files are up-to-date

I reproduced the generator path on a fresh origin/main worktree after #1150 and it wants broad generated drift: remove core/COMPLIANCE.md Hunch rows and strip large hosted client overrides in both SDK clients, plus API reference timestamp/source drift. I am intentionally keeping this PR scoped to #1170 instead of folding that unrelated generated drift into it.

Focused validation passed:

• python3 -m py_compile sdks/python/pmxt/feed_client.py sdks/python/pmxt/__init__.py sdks/python/tests/test_public_exports.py
• direct AST regression check for FeedClient, Ticker, Tickers, OHLCV, Market, FeedMarket, and OracleRound imports + __all__
• git diff --check