feat(rpc): stream ActiveSync Search responses

[TDannhauer]

Summary

• Stream ActiveSync Cmd=Search responses the same way as Cmd=Sync when $conf['activesync']['sync']['streaming'] is enabled.

Motivation

Gmail Android aborts mailbox Search after 30 seconds without response body bytes (SocketTimeout). Search was still fully buffered, so a large IMAP TEXT scan never reached the client in time. See horde/ActiveSync#104.

Changes

• _shouldStreamResponse() treats Search POST like Sync (chunked transfer-encoding, no Content-Length).
• Other commands (GetAttachment, ItemOperations, Find, …) stay on the buffered path.
• Unit test covers Search streaming enablement.

Companion PRs: ActiveSync Search handler keep-alives + IMAP date-chunking, and Core skipping ob_start around keep-alives.

Test plan

• PHPUnit: ActiveSyncStreamingTest (Search streams; ItemOperations/Find do not)
• With streaming on: Gmail Search on a large mailbox returns hits instead of “Problem syncing”
• With streaming = false: Search still uses a buffered Content-Length response