* fix: detect externally opened tabs in --cdp mode (#1037)
Tabs opened outside of agent-browser (e.g. by the user or another CDP
client) were invisible to `tab list` because:
1. `Target.targetCreated` with chrome://newtab/ was filtered by
`is_internal_chrome_target`, and the subsequent `targetInfoChanged`
with the real URL could not update a target that was never tracked.
2. The background drain loop only ran when `request_tracking ||
har_recording` was active, so target events between commands were
silently dropped from the broadcast channel.
Fix: promote untracked targets in `targetInfoChanged` to new targets,
run the background drain unconditionally (guarded by browser presence),
and extract `apply_drained_events` to share target lifecycle processing
(attach, domain filter, iframe sessions) between execute_command and
the background drain.
* refactor: clean up HashSet import and remove call-site duplication
- Import HashSet alongside HashMap instead of using fully-qualified path
- Replace duplicated drain+apply sequence in execute_command with
drain_cdp_events_background call
* style: apply cargo fmt
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Co-authored-by: hyunjinee <leehj0110@kakao.com>
