refactor(services/sftp): deprecate copy option

[Xuanwo]

Which issue does this PR close?

Part of #6708.

Rationale for this change

SFTP enable_copy only controls the advertised copy capability. The SFTP copy implementation does not read this config flag; it streams data through the SFTP client and works with the existing test fixture.

What changes are included in this PR?

This PR enables SFTP copy capability by default, keeps enable_copy as a deprecated no-op compatibility option, and updates generated binding config docs.

Are there any user-facing changes?

No breaking changes. SFTP copy is now advertised by default; endpoints that need to disable copy in tests can use CapabilityOverrideLayer or OPENDAL_TEST_CAPABILITY_OVERRIDES=copy=false.

AI Usage Statement

N/A.