redfish-core: Refactor ThermalMetrics lambda

The handleThermalMetricsHead function contained an inline lambda that
triggered the long-lambda ast-grep rule, requiring an inline
suppression (// ast-grep-ignore: long-lambda).

This commit extracts the lambda into a separate doThermalMetricsHead
function and uses std::bind_front to handle the asynchronous callback.
This aligns the code with the preferred bmcweb pattern for D-Bus
callbacks, improves readability, and allows the removal of the static
analysis suppression.

Change-Id: I656e9d03e0780ac252d8778c825c12def9eac89e
Signed-off-by: Shivraj Toravi <shivrajnt@ami.com>