Normalize legacy ethernet clk_mode to nested clk on config ingest

[bdraco]

Summary

When device-builder ingests ESPHome configs, normalize the legacy flat ethernet clk_mode option into the new nested clk: { mode, pin } form so stored and surfaced configs use the current format. Today we carry clk_mode forward unchanged, which is the reason the ESPHome deprecation removal had to be punted (see esphome/esphome#17114, now targeting 2026.9.0). Auto-mutating on ingest lets ESPHome drop the deprecated option.

Background

ESPHome deprecated the flat clk_mode in favor of:

ethernet:
  type: LAN8720
  mdc_pin: GPIO23
  mdio_pin: GPIO18
  clk:
    mode: CLK_OUT
    pin: 17
  phy_addr: 0

The legacy form encodes the clock pin in the mode string. We already parse it in a couple of places, e.g. _CLK_MODE_PIN_RE and _ETHERNET_HW_FIELDS (which lists both clk and clk_mode) in script/sync_esphome_devices.py, and it shows up in tests/test_sync_esphome_devices_ethernet.py, tests/test_sync_components_typed_schema.py, and the fixture tests/fixtures/yaml_search/ethernet_proxy.yaml.

Mapping

Legacy clk_mode	New clk
GPIO0_IN	mode: CLK_EXT_IN, pin: 0
GPIO0_OUT	mode: CLK_OUT, pin: 0
GPIO16_OUT	mode: CLK_OUT, pin: 16
GPIO17_OUT	mode: CLK_OUT, pin: 17

The rule generalizes: mode is CLK_EXT_IN for *_IN and CLK_OUT for *_OUT; pin is the integer in GPIO<n>_.

Proposed change

On ingest/sync, rewrite any top-level ethernet: block that contains clk_mode so it instead carries clk: { mode, pin } and drops clk_mode. Apply it before we lock featured-component presets and build the pin occupancy map, so downstream code only ever sees the new shape. Where clk_mode is currently special-cased (occupancy parsing, HW preset field set), prefer the nested clk once normalization is in place.

Acceptance criteria

• Incoming configs using clk_mode are stored/surfaced as clk: { mode, pin }; no clk_mode remains.
• Pin occupancy and featured-component presets still resolve the CLK pin correctly via the nested form.
• Tests/fixtures that assert clk_mode behavior are updated to the new shape.
• Verified against a config that uses clk_mode (e.g. the ethernet_proxy.yaml fixture) producing the nested clk output.

Why now

ESPHome punted the clk_mode removal to 2026.9.0 specifically to give device-builder time to handle this (esphome/esphome#17114). Landing this unblocks removing the deprecated option upstream.