feat: implement DumpState for program-aware-fairness

[thc1006]

What type of PR is this?

/kind feature

What this PR does / why we need it:

Adds plugin.StateDumper to the program-aware-fairness plugin so its health can
be inspected through GET /debug/plugins/state, the same way the
inflight-load-producer already exposes its state.

Unlike the other plugins, this one keys its state by program ID, which comes from
a user-controlled request header (x-llm-d-inference-fairness-id) and is
high-cardinality. Dumping those IDs would expose user-controlled, tenant-identifying
values, which docs/plugin_debug.md says to omit. So DumpState reports only
bounded aggregates and no IDs: the number of tracked programs, total in-flight
requests, and Jain's fairness index (which the plugin already computes). That is
the right operational signal for a fairness policy without leaking identities.

Example response for this plugin:

{"totalPrograms":12,"totalInFlight":34,"fairnessIndex":0.92}

A shared jainFairnessIndex helper keeps the index formula in one place so the
dump and the existing metric come from a single snapshot of the program map.

Part of #1755.

Which issue(s) this PR fixes:

Fixes #1798

Release note (write NONE if no user-facing change):

NONE

Testing:

New unit tests, all passing:

• Aggregates (program count, in-flight, fairness index) are reported
• User-controlled program IDs never appear in the dump
• An empty policy returns valid JSON (trivially fair)

[thc1006]

The failing e2e-router (pd) check is the "should report metrics" spec, which is flaky on main at the moment. It fails on the base commit (1fa3803) with the same spec and nothing changed, and this PR only touches the program-aware fairness plugin. Could you re-run that job when you get a chance?