[thermalctld] Add per-component polling intervals from platform.json

[vvolam]

Description

Add support for configurable per-component polling intervals in thermalctld via platform.json. Each component type can specify its own polling rate:

• fan_drawers[0].polling_interval: interval in seconds for fan updates
• psus[0].polling_interval: interval in seconds for PSU thermal updates
• thermals[*].polling_interval: per-thermal sensor interval in seconds

Config entries (without a name key) are inserted at the beginning of fan_drawers / psus arrays. Each thermal device entry with a name can have its own polling_interval.

When any custom thermal intervals are configured, thermals without an explicit polling_interval fall back to the original 60s default instead of running at the fast-loop rate. This prevents unconfigured sensors from being polled too frequently when the main loop interval is reduced to accommodate fast-polling sensors.

The main loop interval adjusts to the minimum of all configured intervals so fast-polling components are serviced on time, while slower components are throttled via per-component timestamp tracking.

Motivation and Context

On platforms with many thermal sensors, polling all sensors every cycle can add unnecessary overhead. Some sensors (e.g., ASIC temperature) need frequent monitoring while others (e.g., ambient temps, SODIMM) can be polled less often. This change allows platform vendors to fine-tune polling rates per component via platform.json without code changes.

How Has This Been Tested?

1. Unit tests: Added tests for _parse_platform_json_polling_intervals(), _should_update_thermal() (including default_interval behavior), and PSU interval gating. All pass locally via pytest.

2. Testbed verification on SN5640: Ran tests on a physical testbed with the following platform.json intervals:

   • fan_drawers: polling_interval: 40
   • psus: polling_interval: 30
   • thermals: ASIC=5s, Ambient Fan Side=10s, Port Side=15s, CPU Pack=20s, SODIMM 2=25s

   Verified:

   • Each component updates at its configured rate (±tolerance for execution overhead)
   • Unconfigured thermals fall back to 60s default
   • Cross-check: fastest thermal (ASIC=5s) updates more often than fans (40s)
   • No errors in thermalctld syslog

3. Backward compatibility: Without any polling_interval in platform.json, behavior is identical to before (all components update every 60s cycle).

Additional Information (Optional)

Example platform.json snippet:

{
    "chassis": {
        "fan_drawers": [
            {"polling_interval": "40"},
            {"name": "drawer1", "fans": [{"name": "fan1"}]}
        ],
        "psus": [
            {"polling_interval": "30"},
            {"name": "PSU 1", "thermals": [{"name": "PSU-1 Temp"}]}
        ],
        "thermals": [
            {"name": "ASIC", "polling_interval": "5"},
            {"name": "CPU Pack Temp", "polling_interval": "20"}
        ]
    }
}

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/azp run

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