fix: sync deprecated flow control env var into feature gate map

[elinacse]

initAdmissionControl reads r.featureGates, which is populated once during parseConfigurationPhaseOne and never revisited. The deprecated ENABLE_EXPERIMENTAL_FLOW_CONTROL_LAYER env var was only applied later, in parseConfigurationPhaseTwo, and only patched rawConfig.FeatureGates — never r.featureGates. So setting the env var logged the deprecation warning but left the EPP silently running the legacy admission controller instead of Flow Control.

As flagged in review, ENABLE_EXPERIMENTAL_FLOW_CONTROL_LAYER was deprecated when GIE 1.5.0 introduced featureGates: [flowControl], and that replacement has been in since router 0.8.0. Rather than fix the env var to work again, this PR removes it entirely ahead of the 0.10.0 release. Use featureGates: [flowControl] in the EndpointPickerConfig instead.

Fixes #2065

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[shmuelk]

The environment variable ENABLE_EXPERIMENTAL_FLOW_CONTROL_LAYER was deprecated in GIE 1.5.0 in favor of the mentioned featureGate.

GIE 1.50. was merged into the llm-d-router in release 0.8.0. I would think that with the upcoming 0.10.0 release the deprecated environment variable ENABLE_EXPERIMENTAL_FLOW_CONTROL_LAYER can be simply deleted.

[elinacse]

@shmuelk removed ENABLE_EXPERIMENTAL_FLOW_CONTROL_LAYER. please review it .

[LukeAVanDrie]

Thanks @elinacse!

Heads up: this conflicts with #2180 (flip the flowControl gate on by default). #2180 rewrites the deprecation log message inside applyDeprecatedEnvFeatureGate, which this PR deletes, so git merge-tree reports a content conflict in cmd/epp/runner/runner.go. feature_gate_test.go merges cleanly.

No coordination needed beyond that. Whichever lands first, the other rebases, and if this one goes first I will just drop my change to that function since it goes away with the variable.

[elinacse]

@LukeAVanDrie Thanks for the review . can you help in merging this PR?

[LukeAVanDrie]

@LukeAVanDrie Thanks for the review . can you help in merging this PR?

I cannot, you will need an approval from someone in llm-d/router-maintainers. I only have perms for review pkg/epp/flowcontrol/... and its plugin impls and fraemwork types under https://github.com/llm-d/llm-d-router/tree/main/pkg/epp/framework/plugins/flowcontrol/... and https://github.com/llm-d/llm-d-router/tree/main/pkg/epp/framework/interface/flowcontrol/...