fix(python): annotate remaining error constructors

[realfishsam]

Summary

• Add explicit -> None return annotations to the remaining typed error constructors.
• Keeps the public runtime behavior unchanged while satisfying the Python type annotation cleanup.

Fixes #731

Test Plan

• python3 -m py_compile sdks/python/pmxt/errors.py sdks/python/pmxt/_exchanges.py
• python3 -m compileall -q sdks/python/pmxt

[realfishsam]

Maintenance automation note: local validation passed for this focused Python typing cleanup:

• python3 -m py_compile sdks/python/pmxt/errors.py sdks/python/pmxt/_exchanges.py ✅
• python3 -m compileall -q sdks/python/pmxt ✅

CI is currently blocked by repository-wide generated client/API reference drift. I checked the generators, but committing their output would include broad hosted-client/docs changes unrelated to this one-file constructor annotation PR, so I left the PR scoped.

[realfishsam]

PR Review: PASS (NOT VERIFIED)

What This Does

Adds explicit -> None return annotations to remaining Python SDK error constructors. This is type-hygiene only and does not change runtime error mapping for SDK consumers.

Blast Radius

Python SDK error classes in sdks/python/pmxt/errors.py only.

Consumer Verification

Before (base branch):
Constructors such as OrderNotFound(order_id, exchange=None) and NetworkError(message, exchange=None) already initialized the same PMXT error codes and retryability flags, but lacked explicit return annotations.

After (PR branch):
The signatures now include -> None while preserving the same super().__init__ arguments and error codes. I verified changed Python syntax with python3 -m py_compile sdks/python/pmxt/errors.py.

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 errors.py)
• Full npm test: FAIL in this checkout during Python SDK collection because pmxt_internal / eth_account are not available.
• Server starts: N/A (type annotation only)
• E2E smoke: N/A (not observable through sidecar HTTP API)

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 type annotation cleanup only.

Risk

I could not run the Python SDK test suite end-to-end in this checkout because generated/internal Python dependencies are unavailable, but the changed constructors are mechanically annotation-only.