feat(client): support for multiple position merges

[cesarenaldi]

Summary

• Add public sync/async execute_transaction(...) methods for custom transaction call batches.
• Add public sync/async merge_multiple_positions(...) helpers for batch market-position or combo-position merges with per-position amount support.
• Reject mixed market/combo merge batches and export typed request shapes for each supported request form.
• Document custom transaction calls and market/combo batch merge usage in the README.

Linear: DEV-344

Verification

• uv run ruff format --check .
• uv run ruff check .
• uv run pyright
• uv run pytest tests/unit/test_sync_relayer_workflows.py tests/unit/test_relayer_position_workflows.py
• uv run pytest currently fails 4 existing wallet-default derivation tests outside this change area: test_secure_client_defaults_wallet_to_current_deposit_wallet, test_secure_client_wallet_defaults_to_current_deposit_wallet_when_omitted, test_async_secure_client_wallet_defaults_to_current_deposit_wallet_when_omitted, test_create_defaults_wallet_to_current_deposit_wallet.