fix(polymarket): take the EOA withdrawal nonce from the write endpoint

[VickyXAI]

Summary

• The EOA withdraw path read its nonce from the public reader, a fallback list of read RPCs, then broadcast through the write RPC. The two can see different pending pools.
• With a reader nonce, the withdrawal could collide with a transaction only the write node has seen. That collision either replaces the other transaction, or gets a nonce too low, which leaves the withdraw guard blocking.
• prepareTransactionRequest now fills the nonce through the wallet's own transport, and that nonce is the one persisted.

Same fix as blockrun-mcp#174, found by an independent review there.

Test plan

• npm test: 847/847
• The withdraw test asserts the nonce comes from the write transport, and that the public reader's getTransactionCount is never called.
• Mutation-checked: putting the reader nonce back makes 5 withdraw tests fail.