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 combo-position merges with per-position amount support.
• Document custom transaction calls and batch combo merge usage in the README.

Linear: DEV-344

Verification

• uv run ruff format --check .
• uv run ruff check .
• uv run pyright
• uv run pytest

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Note

Medium Risk
Changes submit on-chain merge transactions via the authenticated wallet; logic reuses existing dispatch and balance resolution but incorrect amounts or IDs could affect user positions.

Overview
Adds public wallet APIs on sync and async secure clients: execute_transaction submits an ordered list of TransactionCall values through the existing gasless/EOA dispatch path (empty call lists rejected), and merge_multiple_positions batches combo merges in one relayed transaction with optional per-position amount or "max".

Introduces the exported MergePositionRequest typed dict and refactors combo outcome token IDs into derive_combo_outcome_position_ids for reuse when resolving balances before each merge_v2_call. README examples cover custom call batches and multi-combo merges; unit tests assert relayer submit payloads and validation errors.

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