Previously, PSBT `get_key` errors weren’t captured in `SpSendError`,
making failed key lookups hard to diagnose. This commit introduces a new
`KeyError` variant to represent any failure from the `K: GetKey`
provider and updates `SpSendError`’s `Display` impl with a generic error
message.

Note: the `GetKey` error type lacks a `Display` implementation, so the
underlying error can’t be shown; `KeyError` serves as a marker for
private‐key retrieval failures.

Introduce two new variants to the SpSendError enum:
- MissingDerivations: For cases where the PSBT lacks required key
  derivations.
- MissingOutputs: For scenarios where output counts do not match
  expectations.

These errors enable more precise handling of derivation and output
validation issues in the send process, without altering existing
behavior. This is a pure addition to improve error granularity for
future refactors and fixes.

No breaking changes; existing code remains unaffected.

The create_silentpayment_scriptpubkeys API no longer returns a Result
since it cannot fail on its own.

- create_silentpayment_scriptpubkeys never produces an error internally,
  so returning Result was misleading
- eliminates needless panic points and clarifies that failure cannot
  occur at this stage
- streamlines code paths and makes error propagation more consistent
  elsewhere

Extract prevout script retrieval logic into a standalone function. This
separates the logic for getting script_pubkey from either witness_utxo
or non_witness_utxo into its own function without changing behavior.

No changes in derive_sp logic.

Extract taproot private key derivation logic into a standalone function.
This separates the taproot key handling including parity adjustment and
tap tweaking into its own function.

…e_sp

Extract non-taproot private key derivation logic into a standalone
function.
This separates the BIP32 and public key-based key retrieval logic.

Extract transaction input building logic into a standalone function.

This isolates the logic for constructing a complete TxIn with witness
from PSBT input data.

Extract input data collection logic into a standalone function.

This separates the logic for gathering scripts with secrets and finding
the lexicographically minimum outpoint.

Extract PSBT output update logic into a standalone function.
This separates the logic to update PSBT outputs with silent payment
data.

Replace inline logic in derive_sp with calls to extracted functions.
This completes the refactoring by using collect_input_data and
update_outputs.