Commit Message: xds-failover: disable moving to primary after fallback responds
Additional Description:
In #35591 Envoy's implementation was modified to allow Envoy move to the primary xDS source, even after the fallback successfully responded.
This PR adds a runtime guard that reverts this behavior. This will be a temporary runtime guard that is introduced to allow testing of the new feature and will be later removed.
Note that there's no change to the current Envoy behavior, unless the runtime guard envoy.reloadable_features.xds_failover_to_primary_enabled is explicitly set to false.

Risk Level: low - behind a runtime guard.
Testing: Added unit and integration tests.
Docs Changes: N/A
Release Notes: Added an entry.
Platform Specific Features: N/A
Runtime guard: envoy.reloadable_features.xds_failover_to_primary_enabled was introduced that is set to true by default, and keeps the current behavior.

Request variables

Key	Value
ref	1309a05
sha	2cc00f6
pr	36386
base-sha	ce17605
actor	@adisuissa
message	xds-failover: disable moving to primary after fallback responds...
started	1727808211.252127
target-branch	main
trusted	false

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Envoy version (as used in this CI run)