Normalize rack-manager alert severity to upper case at ingest

[chinmoy-nexthop]

Why:

Redfish clients send the SonicSeverity enum in mixed case (Normal, Minor,
Major, Critical), but the platform STATE_DB schema and its consumers compare
against upper-case constants: thermalctld reads SYSTEM_LEAK_STATUS and bmcctld
dispatches on LEAK_CONTROL_POLICY with checks such as severity equals CRITICAL.
sonic-dbus-bridge stored whatever the client sent, verbatim, so a standards
compliant "Critical" leak alert was written to RACK_MANAGER_ALERT as "Critical"
and the consumer comparison silently missed it. The result was a critical leak
that produced no dispatch and no visible error, which is a safety-relevant
failure mode rather than a cosmetic one.

How:

The casing is fixed once, at the ingest boundary, instead of being worked
around in every consumer:
- Added a small normalizeSeverity helper in rack_manager_receiver.cpp that
  upper-cases the string with an unsigned-char-safe std::toupper, with a
  comment recording why the platform schema requires it.
- effectiveSeverity now returns the normalised value for every severity that
  is present, so both SubmitAlert and SubmitTelemetry payloads are covered by
  the single unified extraction path.
- The fallback used when no severity is supplied anywhere along the
  inheritance chain changed from "Normal" to "NORMAL", and the accompanying
  warning text was aligned with it, so the default takes the same shape as a
  supplied value.
- The behaviour is documented in the OEM extension README so clients know the
  stored value is upper-cased.
The leak-to-action decision itself was deliberately left in bmcctld's policy;
the bridge only owns the RACK_MANAGER write, so no policy moved here.

How to verify:

- POST a SubmitAlert payload carrying "Severity": "Critical" and confirm the
  stored row holds CRITICAL, with
  redis-cli -n 6 HGETALL "RACK_MANAGER_ALERT|Rack_level_leak".
- Repeat with lower-case "critical" and mixed-case "CrItIcAl" and confirm both
  land as CRITICAL.
- POST a payload with the Severity key omitted and confirm the row holds NORMAL
  and the bridge logs the missing-severity warning.
- Confirm bmcctld now dispatches on the critical alert that previously produced
  no action.
- make unit-test and make test, to confirm nothing else regressed.

[mssonicbld]

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[chinmoy-nexthop]

@judyjoseph

[judyjoseph]

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[judyjoseph]

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[judyjoseph]

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