Expose RTK status sensors for ecovacs mowers (GOAT)

[Beennnn]

Proposed change

Adds 8 diagnostic sensors backed by the new `Capabilities.rtk` from `deebot-client` (currently only the GOAT A1600 RTK):

• `rtk_rover_satellites_visible` / `rtk_base_satellites_visible`
• `rtk_rover_satellites_used` / `rtk_base_satellites_used`
• `rtk_rover_signal_score` / `rtk_base_signal_score` (0-100)
• `rtk_rover_occlusion_rate` / `rtk_base_occlusion_rate` (default disabled)

Same data the official Ecovacs Home app shows under Settings > RTK.

Library counterpart: DeebotUniverse/client.py#1587. Manifest stays at `deebot-client==18.3.0` here; bump will be a one-line follow-up commit on this branch when 18.4.0 ships #1587. CI failures (pylint/mypy on the `RtkEvent` import) are expected until then.

Type of change

• Dependency upgrade
• Bugfix (non-breaking change which fixes an issue)
• New integration (thank you!)
• New feature (which adds functionality to an existing integration)
• Deprecation (breaking change to happen in the future)
• Breaking change (fix/feature causing existing functionality to break)
• Code quality improvements to existing code or addition of tests

Additional information

• This PR fixes or closes issue: fixes #
• This PR is related to issue:
• Link to documentation pull request:
• Link to developer documentation pull request:
• Link to frontend pull request:

Checklist

• I understand the code I am submitting and can explain how it works.
• The code change is tested and works locally.
• Local tests pass. Your PR cannot be merged unless tests pass (no local HA Core venv on this machine; entity descriptions mirror the existing network sensors pattern; relying on CI once the deebot-client release is out)
• There is no commented out code in this PR.
• I have followed the development checklist
• I have followed the perfect PR recommendations
• The code has been formatted using Ruff (`ruff format homeassistant tests`)
• Tests have been added to verify that the new code works. (Sensor entity tests will follow once the deebot-client release with RtkEvent is shipped and the manifest pin is bumped.)
• Any generated code has been carefully reviewed for correctness and compliance with project standards.

If user exposed functionality or configuration variables are added/changed:

• Documentation added/updated for www.home-assistant.io

If the code communicates with devices, web services, or third-party tools:

• The manifest file has all fields filled out correctly.
  Updated and included derived files by running: `python3 -m script.hassfest`.
• New or updated dependencies have been added to `requirements_all.txt`.
  Updated by running `python3 -m script.gen_requirements_all`.
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[home-assistant]

Hey there @mib1185, @edenhaus, @Augar, mind taking a look at this pull request as it has been labeled with an integration (ecovacs) you are listed as a code owner for? Thanks!

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[Beennnn]

Status update — this PR is blocked on DeebotUniverse/client.py#1587 (feat: parse getRTK for mower devices) which provides RtkEvent and Capabilities.rtk.

Once #1587 is merged + new release, I'll bump manifest.json here. #1587's CI is green and awaiting review.