[Mellanox] Fix BMC import issue

[stephenxs]

Why I did it

After lazy BMC initialization (#26901), from .bmc import BMC lived only inside initialize_components(). initialize_bmc() still calls BMC.get_instance() in the chassis module scope, so BMC was never defined there. That breaks BMC CLIs on OpenBMC platforms, for example:

• sudo show platform bmc summary → Error retrieving BMC information: name 'BMC' is not defined
• sudo show platform bmc eeprom → Error retrieving BMC EEPROM information: name 'BMC' is not defined

The import must run in the function that uses BMC, while keeping lazy load so non-BMC platforms do not pull in BMC/Redfish at chassis import time.

Work item tracking

• Microsoft ADO (number only):

How I did it

• Moved from .bmc import BMC from initialize_components() into initialize_bmc() in platform/mellanox/mlnx-platform-api/sonic_platform/chassis.py.
• Left the DeviceDataManager.is_platform_with_bmc() gate in initialize_components() unchanged; only the import site moved.
• Lazy-load behavior is unchanged: bmc.py is still loaded only when BMC initialization runs on BMC-capable platforms.

How to verify it

1. Unit tests (from repo root):

   cd platform/mellanox/mlnx-platform-api
   pytest tests/test_chassis.py tests/test_bmc.py -v

2. **On a BMC-capable DUT** (e.g. SN6600 LD with bmc.json present):

```bash
sudo show platform bmc summary
sudo show platform bmc eeprom

Expect BMC fields (manufacturer, model, serial, etc.), not name 'BMC' is not defined.

Which release branch to backport (provide reason below if selected)

• 202305
• 202311
• 202405
• 202411
• 202505
• 202511

Tested branch (Please provide the tested image version)

Description for the changelog

Link to config_db schema for YANG module changes

A picture of a cute animal (not mandatory but encouraged)

[mssonicbld]

/azp run Azure.sonic-buildimage

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Azure Pipelines successfully started running 1 pipeline(s).

[mssonicbld]

/azp run Azure.sonic-buildimage

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[stephenxs]

/azpw run

[mssonicbld]

⚠️ Notice: /azpw run only runs failed jobs now. If you want to trigger a whole pipline run, please rebase your branch or close and reopen the PR.
💡 Tip: You can also use /azpw retry to retry failed jobs directly.

Retrying failed(or canceled) jobs...

[mssonicbld]

Retrying failed(or canceled) stages in build 1130678:

✅Stage Build:

• Job marvell_prestera_armhf: retried.

[benle7]

LGTM!