[FEATURE] Dynamic Mower Dropdown Selector

[WhyLev]

Replace the manual text input for mower_serial with a dynamic dropdown selector in all integration services to eliminate typos and improve user experience.

Technical Changes

Update services.yaml: Change the mower_serial selector from text to select with a dropdown mode:

• Rename Label: Change the UI field name from "Serialnumber" to "Mower".
• Backward Compatibility: Keeping custom_value: true ensures existing YAML automations, scripts, and manual testing still work perfectly.
• Fallback Logic: Retain the existing find_instance_for_mower_service_call() behavior (automatically targets the single mower if only one is configured).

Key Benefits

• Zero Typos: Completely eliminates 16-character entry errors.
• Multi-Mower Friendly: Instantly lists all available mowers instead of forcing users to look up serial numbers.
• Native UX: Aligns perfectly with Home Assistant's modern UI patterns.

[kimzeuner]

What do you think of this Link to my branch

For functional tests the relevant changes are:

1. Added mower_entity to all service definitions in services.yaml

   • Allows selecting a mower from a dropdown instead of entering the serial number manually.

2. Added CONF_MOWER_ENTITY to const.py

3. Updated all service schemas

   • Added:

     vol.Optional(CONF_MOWER_ENTITY): cv.entity_id

4. Updated find_instance_for_mower_service_call()

   • Services can now resolve the mower instance from a selected lawn_mower entity.
   • Existing mower_serial support remains unchanged for backward compatibility.

[WhyLev]

Added in Pullrequest #317