fix(python-sdk): export NotSupported at package root

[realfishsam]

Summary

• Re-export NotSupported from the Python package root
• Include NotSupported in pmxt.__all__ for parity with the TypeScript SDK package exports

Fixes #985

Test Plan

• python3 -m py_compile sdks/python/pmxt/__init__.py sdks/python/pmxt/errors.py
• Static export smoke: verified NotSupported import and __all__ entries in sdks/python/pmxt/__init__.py
• Runtime import pmxt is blocked in this checkout by missing generated pmxt_internal, so no package import smoke was run.

[realfishsam]

Independent validation for the PR-scoped change passed: python3 -m py_compile sdks/python/pmxt/__init__.py sdks/python/pmxt/errors.py and static export checks for NotSupported import/__all__. Current red generated-client/API-reference checks are the repo-wide hosted-client generator drift (running the generator removes hosted-mode client helpers unrelated to this PR), so I did not commit that broad generated drift into this focused fix.

[realfishsam]

PR Review: PASS (NOT VERIFIED)

What This Does

Exports NotSupported from the Python package root so users can import it with from pmxt import NotSupported. This aligns package-root exports with the SDK error surface.

Blast Radius

Python SDK package exports in sdks/python/pmxt/__init__.py only.

Consumer Verification

Before (base branch):
NotSupported existed in pmxt.errors but was not imported into package root or listed in __all__.

After (PR branch):
sdks/python/pmxt/__init__.py imports NotSupported and includes it in __all__. Changed Python syntax passes py_compile.

Test Results

• Build: PASS (npm run build --workspace=pmxt-core)
• Core unit tests: PASS (27 suites / 652 tests passed; 1 suite / 3 tests skipped)
• Python syntax: PASS (py_compile on changed file)
• Full npm test: FAIL in this checkout during Python SDK collection because pmxt_internal / eth_account are not available.
• Server starts: N/A (package export only)
• E2E smoke: Not run because importing the package root in this checkout is blocked by missing generated pmxt_internal

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 -- additive Python export.

Risk

Runtime import could not be executed in this checkout due missing generated Python package, but the diff is a direct import/export addition.