refactor(gateway): extract GatewayPlatformMixin from run.py (slice 25 of #54962)

[andrexibiza]

Related #54962 #55138

What

Extract the platform-adapter lifecycle cluster out of GatewayRunner in gateway/run.py (26.8k-line god file) into a new gateway/platform_mixin.py module as GatewayPlatformMixin — 34 methods / ~1,668 LOC moved byte-verbatim (zero behavior change).

This is slice 25 of the large-file decomposition campaign (Wave 3, mixin lifts), following the codebase's established mixin pattern (gateway/authz_mixin.py, gateway/kanban_watchers.py, gateway/slash_commands.py).

What moved — the adapter lifecycle cluster:

• connect/disconnect/teardown timeouts (_await_adapter_cleanup_with_timeout, _safe_adapter_disconnect, _bounded_adapter_teardown, _adapter_disconnect_timeout_secs, _platform_connect_timeout_secs, _connect_adapter_with_timeout, _connect_initial_adapter_with_timeout)
• fatal-error handling (_handle_adapter_fatal_error*)
• reconnect watcher + secondary-profile (multiplex) startup (_platform_reconnect_watcher, _start_secondary_profile_adapters, _start_one_profile_adapters, _configure_profile_adapter, _run_secondary_profile_reconnect, _schedule_secondary_profile_reconnect, ...)
• adapter credential/listener claim conflict detection (_adapter_credential_claim, _adapter_listener_claim, _adapter_credential_fingerprint)
• handoff watcher (_handoff_watcher, _process_handoff), profile-name resolution, systemd watchdog, auth-check factory

Why

gateway/run.py is a ~26.8k-line god file. GatewayRunner is the largest class; the platform-adapter cluster is a cohesive, self-contained concern (only self.* + neutral helpers) that lifts cleanly into a mixin, exactly like the three existing ones. GatewayRunner now reads GatewayPlatformMixin first in its bases.

How

• New module gateway/platform_mixin.py: class GatewayPlatformMixin with the 34 methods moved verbatim (docstrings, comments, and bodies byte-identical to the pre-move source).
• gateway/run.py: methods deleted from the class body; GatewayPlatformMixin added FIRST in GatewayRunner's bases; module-attribute import added to the from gateway.* block.
• Zero behavior change: self.* calls resolve unchanged via the MRO. Neutral dependencies import at module top. Helpers that stay in run.py (_profile_runtime_scope, _reconnect_backoff, _dispose_unused_adapter, _platform_has_bot_credential, _own_policy_open_startup_violation, MultiplexConfigError, SecondaryPortBindingConfigError, the timeout defaults, get_hermes_home, GatewayRunner itself for a staticmethod self-reference) are imported lazily inside the using method (from gateway.run import ... at call time) — the sibling-mixin pattern that avoids the import cycle. The module-level logger = logging.getLogger("gateway.run") preserves the original logger name so log records are unchanged.
• @staticmethod decorators preserved on _adapter_credential_claim / _adapter_listener_claim / _adapter_credential_fingerprint.
• The "Kanban board watchers" section comment (a class-body marker for the already-extracted kanban mixin) stays in run.py.

How to test

# Import smoke (mixin FIRST in MRO, methods resolve through it):
python -c "import gateway.run; assert hasattr(gateway.run.GatewayRunner, '_await_adapter_cleanup_with_timeout'); assert hasattr(gateway.run.GatewayRunner, '_create_adapter'); assert hasattr(gateway.run.GatewayRunner, '_profile_name_for_source')"

# Targeted suites (all green on this change):
python -m pytest tests/gateway/test_bounded_adapter_teardown.py tests/gateway/test_safe_adapter_disconnect.py tests/gateway/test_platform_reconnect.py tests/gateway/test_platform_reconnect_fd_leak.py tests/gateway/test_runner_fatal_adapter.py tests/gateway/test_systemd_watchdog_lifecycle.py -q --no-header -p no:cacheprovider
python -m pytest tests/gateway/test_64674_multiplex_primary_token_scope.py tests/gateway/test_multiplex_adapter_registry.py tests/gateway/test_multiplex_pairing_stores.py tests/gateway/test_multiplex_phase0.py tests/gateway/test_multiplex_profile_authz.py tests/gateway/test_handoff_watcher_async_db.py tests/gateway/test_handoff_thread_session_key.py tests/gateway/test_profile_resolution.py tests/gateway/test_platform_registry.py -q --no-header -p no:cacheprovider
python -m pytest tests/gateway/test_runner_startup_failures.py tests/gateway/test_startup_restart_race.py tests/gateway/test_telegram_network_reconnect.py tests/gateway/test_own_policy_startup_gate.py tests/gateway/test_75349_whatsapp_multiplex_secret_scope.py tests/gateway/test_api_server_multiplex_secret_scope.py tests/gateway/test_multiplex_credential_isolation.py tests/gateway/test_multiplex_lifecycle.py tests/gateway/test_multiplex_background_task_scope.py tests/gateway/test_multiplex_http_routing.py tests/gateway/test_multiplex_api_server_routing.py -q --no-header -p no:cacheprovider
python -m pytest tests/gateway/test_shutdown_cache_cleanup.py tests/gateway/test_qqbot_scope_paths.py tests/gateway/test_adapter_connect_is_reconnect_contract.py tests/gateway/test_adapter_startup_secret_scope.py tests/gateway/relay/test_handoff_relay_aliasing.py tests/gateway/test_kanban_notifier.py tests/gateway/test_telegram_topic_mode.py tests/gateway/test_loop_liveness_watchdog.py tests/gateway/test_startup_no_eager_platform_install.py tests/gateway/test_slack_socket_reconnect_heal.py -q --no-header -p no:cacheprovider

Test results (this PR): 32 + 67 + 46 + 121 + 132 passed = 398 passed, 1 xfailed, 0 failures across the moved-method suites. One unrelated pre-existing failure (test_api_server.py::TestHealthDetailedEndpoint::test_health_detailed_returns_ok) reproduces identically on pristine main (stash-proven) and is not touched by this change. git diff --check clean; check-windows-footguns.py clean on both changed files.

Shrink

• gateway/run.py: 26,823 → 25,205 lines (−1,618)
• New gateway/platform_mixin.py: 1,735 lines (34 methods)

Scope note

Pure mechanical move — no logic touched, no signatures changed. Byte-verbatim verified per-method against the pre-move source (the only inserted lines are the lazy from gateway.run import ... statements inside the 9 methods that reference run.py-local helpers, matching the sibling-mixin pattern). The full suite carries ~250 pre-existing environment failures unrelated to this change; only targeted suites were run here.

Part of #54962
Part of #55138

Part of #78207
Part of #78647
Part of #78791