[action] [PR:750] [xcvrd] Separate VDM basic and statistic observables

[mssonicbld]

This PR must be merged along with sonic-net/sonic-platform-common#624

Description

Currently, xcvrd performs freeze operation before reading ALL VDM observables,
then unfreezes after reading. This approach has following limitations:

1. Modules supporting only basic VDM observables (instantaneous values) may not
   support freeze/unfreeze functionality
2. Freeze operations are expensive and unnecessary for basic observables

Motivation and Context

This PR restructures the VDM monitoring flow in DOM manager to optimize freeze
operations and correctly handle different observable types.

CHANGES IMPLEMENTED:

1. Three-Step VDM Monitoring Flow (dom/dom_mgr.py)

   Step 1: Conditional Freeze for Statistics/PM (only if NOT in lpmode)

   • Check: need_freeze = vdm_statistic_supported AND not_lpmode
   • If need_freeze:
     a) Freeze VDM/PM statistics
     b) Read VDM statistic observables (if supported)
     c) Read PM data
     d) Unfreeze VDM/PM statistics

   Step 2: Read VDM Basic Observables

   • Always read, no freeze needed
   • Works even in lpmode
   • get_vdm_real_values_basic() → instantaneous values
   • Merged with statistic observables from Step 1

   Step 3: Post Merged Data to DB, then Read COR Flags

   • Post combined basic + statistic observables to DB
   • Read COR flags last (ensures statistics captured before flags clear)

How Has This Been Tested?

Following tests were executed to ensure that the VDM related tables are updated in line with expectation

1. Optic supporting only VDM basic observables
2. Optic supporting both VDM basic and statistic observables
3. Optic supporting both VDM basic and statistic observables + CCMIS optic
4. Optics not supporting VDM and PM (800G LPO optics)
5. While module is in lpmode, ensure that the VDM statistical observables are not being updated

• Please note that the last_update_time timestamp will be updated as VDM basic observables would still be periodically updated while the transceiver is in low power mode

6. While CCMIS module is in lpmode, ensure that the VDM statistical observables + PM data is not being updated
7. Flat memory optics
8. Test SFP OIR
   a. Swap CCMIS optic with non-VDM supported optic
   b. Swap non-VDM supported optic with DAC
   c. Swap DAC with CCMIS optic

Additional Information (Optional)

MSFT ADO - 34227059

[mssonicbld]

Original PR: #750

[mssonicbld]

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