[sonic_xcvr] Cache static CMIS support/identity fields to optimize EEPROM reads

[aditya-nexthop]

Description

Cache static CMIS support/identity/capability fields so they are read from the module EEPROM once and reused, instead of being re-read on every DOM/status poll cycle.

Changes:

• Cache static getters by applying the @read_only_cached_api_return
  decorator (caches until the getter returns a non-None, non-empty value) to
  the CMIS advertisement getters that expose static, read-only fields

• Add a cached _get_tx_bias_scale_raw() helper. The TX bias scaling
  exponent is a static per-module advertisement, but it was read from EEPROM on
  every get_tx_bias() and get_transceiver_threshold_info() call. Both paths
  now go through the cached helper.

• Return None on failed reads instead of freezing a default. Because the
  cache only stores non-None results, getters now return the raw None on a
  transient read failure so the value is re-read on the next call rather than
  caching a wrong default.

• Update callers for the new get_supported_power_config contract
  (c_cmis.py): set_tx_power() and get_transceiver_info() now use
  self.get_supported_power_config() or (None, None) to safely unpack when the read fails.

Motivation and Context

These fields (support flags, module identity/interface, and capability
advertisements) are static for the lifetime of a plugged-in module, yet they
were being re-read from the EEPROM on every DOM/status polling cycle. On a
platform with many transceivers this adds redundant I2C/EEPROM traffic and
latency to the hot polling loop.

Caching them behind @read_only_cached_api_return reads each field once and serves subsequent calls from memory, cutting EEPROM reads in the per-cycle DOM path. Returning None on a failed read (instead of a hard-coded False / (None, None)) ensures a transient read failure is not cached permanently — the cache re-reads on the next call once the module responds.

How Has This Been Tested?

• Updated existing unit tests in tests/sonic_xcvr/test_cmis.py to reflect the
  new None-on-failed-read behavior:
  • test_get_diag_page_support: a None read now expects None (previously
    False).
  • test_is_cdb_supported: a None read now expects None (previously
    False).
• Covered by the caching unit tests in tests/sonic_xcvr/test_cmis_cache.py,
  which validate that decorated getters are cached and can be cleared.
• Full sonic_xcvr pytest suite run locally.

Additional Information (Optional)

[mssonicbld]

/azp run

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