[sai] Adopt pre-existing port serdes on SAI_STATUS_ITEM_ALREADY_EXISTS

[AnantKishorSharma]

Summary

On cold-boot config apply, SaiPortManager may recreate a port via
changePortByRecreate() when a create-only attribute (e.g. lane or speed)
differs from the port the SAI switch created by default. The recreate removes
the old SAI port and adds a new one, which programs a new port serdes.

Some SAI implementations do not remove the port serdes object when its parent
port is removed. On the subsequent recreate,
programSerdes() -> store.setObject() -> create_port_serdes then fails with
SAI_STATUS_ITEM_ALREADY_EXISTS. Because the SaiObjectStore holds no handle
for the serdes (on XGS the serdes is not reloaded from the adapter beforehand),
the error propagates as an uncaught SaiApiError and aborts the HW agent during
initialization.

Fix

When the serdes create returns SAI_STATUS_ITEM_ALREADY_EXISTS, adopt the
pre-existing hardware serdes: reload it into the store from the port's
SerdesId and re-apply the desired attributes. This reuses the
getAttribute(SerdesId) + reloadObject() idiom already used in
programSerdes() on the non-XGS reload path. On implementations that remove the
serdes together with its port, create never returns ITEM_ALREADY_EXISTS, so
there is no behavior change. Both serdes create sites go through a small
setSerdesObject() helper.

Test Plan

Reproduced on a xgs platform running the agent HW test in multi_switch
mode: the HW agent aborted during init in
SaiPortManager::programSerdes() -> changePortByRecreate() with
[port] Failed to create sai entity PortSerdes...: ITEM ALREADY EXISTS. With
this change the existing serdes is adopted instead of aborting.

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