docs: sync external-signer.md with current external signer flow #33947

Updated external-signer.md to match the current implementation. The signer network selection is passed as --chain rather than --testnet, and transaction signing is performed via stdin using the signtx command with JSON output containing a psbt field or error. Wallet creation for external signers requires external_signer=true with private keys disabled, so the example now uses named arguments. Spending with an external signer goes through the new send/sendall RPCs which build a PSBT, call the signer, and finalize when signatures are sufficient; sendtoaddress is not applicable for such wallets and also returns a plain txid for pk-enabled wallets rather than a complete field. The getdescriptors call is documented with the --account parameter and return shape including receive and internal arrays. Examples for displayaddress and createwallet are corrected, enumeratesigners now shows an optional name field, and references to bitcoin-qt replace the non-existent “bitcoin node” and “bitcoin rpc”.

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The following sections might be updated with supplementary metadata relevant to reviewers and maintainers.

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Code Coverage & Benchmarks

For details see: https://corecheck.dev/bitcoin/bitcoin/pulls/33947.

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Thanks, however there is an existing PR at #33765. I'd suggest reviewing there first.

Thanks, however there is an existing PR at #33765. I'd suggest reviewing there first.

Oh, actually didn't notice that PR

I tried to include some of your improvements in #33765. Feel free to comment or collaborate.