[source-sftp-bulk] Support passphrase-protected private keys

> 🤖 *This issue was generated by an AI Agent.*

## What's missing

Private keys encrypted with a passphrase cannot be used — there is no field to supply the passphrase and no code path to decrypt the key.

## Verified on `master`

`passphrase` does not appear anywhere in the `source_sftp_bulk` package. `client.py` loads keys via `key_class.from_private_key(io.StringIO(private_key))` with no password argument.

Note that non-RSA key *types* are supported (RSA, Ed25519, ECDSA, DSS — `client.py` lines 20-22, added in 1.9.1 via #75967). That is a different feature from unlocking an encrypted key.

## Origin

Closed as stale: #71309 (@marcos-t21), reviewed 2026-01-20 with no author response.

Filed to keep a record of the gap. The PR was closed for inactivity, not on merit — credit to the original contributor if this is picked up.

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**Internal Tracking:** https://github.com/airbytehq/oncall/issues/13175