Warn and leave Powerwall gateway host blank when discovery fails

[Bre77]

Breaking change

Proposed change

The energy-site subentry flow discovers the local Powerwall gateway address via
find_gateway_address() before showing the credentials form. When discovery
returned no address or raised, the form defaulted the host to
DEFAULT_GATEWAY_HOST (192.168.91.1, the setup-AP address) and logged only at
debug — silently pointing the user at the wrong host for a normally-connected
gateway. The form now leaves the host field blank and logs a warning on both the
no-address and error paths, so the user enters the gateway host instead of
inheriting a misleading default.

The tests pin Home Assistant's side of the gateway-discovery contract at the
find_gateway_address() boundary, not the library's address decoding:

• a discovered address pre-fills the credentials host default;
• given a realistic two-interface networking_status, the resolved active-route
  LAN address is surfaced rather than the gateway-internal one (the interface
  selection itself is the library's responsibility);
• both the None and exception (ClientError / TeslaFleetError) paths leave
  the field blank and emit a warning.

The fixture is Teslemetry-format networking_status (dotted-quad string
addresses; Tesla Fleet's own API serves uint32 ints), with all identifying
values replaced by synthetic ones.

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:

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