test: cover sync SecureClient market order placement

[cesarenaldi]

Summary

• add shared deposit wallet key/address fixtures for integration tests
• add metered sync SecureClient.create(...).place_market_order(...) coverage
• keep the test using derived credentials so it exercises the default sync client path

Refs #136

Verification

• uv run pytest tests/integration/test_clob_orders.py::test_sync_secure_client_create_places_market_order (skipped unless metered tests are enabled)
• uv run ruff check tests/integration/conftest.py tests/integration/test_clob_orders.py
• uv run ruff format --check tests/integration/conftest.py tests/integration/test_clob_orders.py
• uv run pyright tests/integration/conftest.py tests/integration/test_clob_orders.py

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Note

Low Risk
Test-only changes; the new metered test can place real orders when enabled but does not modify production SDK behavior.

Overview
Adds shared deposit_wallet_private_key and deposit_wallet_address fixtures in integration conftest, and wires deposit_wallet_client through them instead of reading env vars inline.

Adds a metered sync test test_sync_secure_client_create_places_market_order that uses SecureClient.create(...) as a context manager, lists positions, and places a live FAK market order (sell existing Yes inventory or buy minimum size), asserting an AcceptedOrder with an expected status—parallel to the existing async market-order coverage.

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