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### Description of PR
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Summary:
Add `platform.json` support to the `port_config_gen` Ansible module (fanout port config generator). When `hwsku.json` exists and `platform.json` contains a non-empty `interfaces` section, use them for port config lookup. Fall back to `port_config.ini` otherwise.

### Type of change

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- [ ] Bug fix
- [x] Testbed and Framework(new/improvement)
- [ ] New Test case
    - [ ] Skipped for non-supported platforms
- [ ] Test case improvement

### Back port request
- [ ] 202205
- [ ] 202305
- [ ] 202311
- [ ] 202405
- [ ] 202411
- [ ] 202505
- [x] 202511

### Approach
#### What is the motivation for this PR?
This is part of the larger effort to support generic HWSKUs. One of the steps toward that goal is removing the dependency on `port_config.ini` ([sonic-net/sonic-buildimage#24494](sonic-net/sonic-buildimage#24494)). The `port_config_gen` Ansible module currently relies solely on port_config.ini for port configuration, so it needs to be updated to also support `platform.json`.

The changes are in line with the existing port config resolution logic in [device_info.py](https://github.com/sonic-net/sonic-buildimage/blob/master/src/sonic-py-common/sonic_py_common/device_info.py#L443).

#### How did you do it?
- Add JSON-based port config lookup: use `platform.json` when `hwsku.json` exists and `interfaces` section in `platform.json` is populated. Use `port_config.ini` otherwise
- Define `_read_from_json()` to parse `platform.json` into the same alias-keyed dict format as `_read_from_port_config()`
#### How did you verify/test it?
Tested on T1 and LT2 platforms: ran `port_config_gen.py` via an Ansible stub and verified the new `platform.json` + `hwsku.json` path generates the same port config as the legacy `port_config.ini` path.
#### Any platform specific information?

#### Supported testbed topology if it's a new test case?

### Documentation
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Signed-off-by: Sonic Build Admin <sonicbld@microsoft.com>