[BMC] Align is_bmc_supported check for the new infra

[benle7]

What I did

The BMC support check in generate_dump was aligned with the new infrastructure flow
(sonic-net/sonic-buildimage#26544).
Instead of checking for platform-local bmc.json assumptions, the logic now validates host-side role and runtime BMC data from the canonical source used by the system.

How I did it

• Updated generate_dump BMC detection to use device_info.is_switch_host() together with
  device_info.get_bmc_data().
• Removed reliance on legacy platform-folder bmc.json existence behavior for this path.
• Kept the generate_dump flow behavior intact while aligning detection to runtime
  /etc/sonic/bmc.json-based APIs.

How to verify it

• Run show techsupport (or generate_dump) on:
  • a switch-host platform with valid BMC data and verify BMC-related flow executes.
  • a non-switch-host / missing-BMC-data platform and verify BMC path is skipped.

Previous command output (if the output of a command-line utility has changed)

New command output (if the output of a command-line utility has changed)

[mssonicbld]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[mssonicbld]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).