Remove dashboard_import.import_config from esphome once the container dashboard bundle no longer needs it

`esphome.components.dashboard_import.import_config` has no caller left in the esphome tree. The Device Builder adopts `?full_config` devices in-house since esphome/device-builder#2720, and the legacy dashboard was deleted from the tree. The only remaining consumers are deployed Device Builder versions older than esphome/device-builder#2720 (the ESPHome desktop app, or a manually run Docker image, pinned to an older build), which still delegate `?full_config` adoption to it. The function only made sense while the dashboard was in tree.

Removing it also drops `ruamel.yaml` and `ruamel.yaml.clib` from esphome's requirements; both lines are annotated `# dashboard_import` and nothing else imports them.

Not yet: wait until the Device Builder bundled in the desktop app and the Docker image is past #2720 and older installs have aged out. Revisit around March 2027.

Order of operations when it is time:

1. Drop `tests/test_device_yaml.py::test_generate_adoption_yaml_matches_dashboard_import` in esphome/device-builder; it imports `import_config` to pin the adoption shape and would break that repo's esphome-dev CI job.
2. In esphome/esphome remove `import_config`, its unit tests in `tests/unit_tests/test_dashboard_import.py`, and the two `ruamel.yaml` requirement lines. The component's config schema, `to_code`, and the `dashboard_import` mDNS TXT stay; only the dashboard-side helper goes.
3. Remove `esphome.components.dashboard_import` from the cold-import guard list in `tests/test_cold_import_floor.py` in esphome/device-builder, or leave it, since a missing module trivially stays cold.

First verify that the Device Builder bundled in the desktop app and Docker image is past #2720 and older installs have aged out; this issue is explicitly deferred until around March 2027. Then inspect tests/test_device_yaml.py, esphome/components/dashboard_import, tests/unit_tests/test_dashboard_import.py, requirements, and tests/test_cold_import_floor.py. Done means removing only the dashboard-side helper, its tests and dependencies, while retaining the component schema, to_code, and mDNS TXT behavior.

Written by the indexing model from the issue text.