feat: ✨ Session budget with HITL pause mode

[evaline-ju]

Summary

• Rename token-budget to session-budget. The plugin is not limited to observing/budget enforcement tokens but also calls and durations.
• Pause mode feature: new on_exceed: 'pause' mode — HITL approval: POST to a webhook on breach and block the request until it responds {"action":"approve"} or pause_timeout fires.
  • New config: pause_webhook, pause_timeout (30s), pause_timeout_action (deny|allow), pause_grace_period (5m)
  • After approval, subsequent requests skip the webhook for the pod-local grace window
  • Concurrent requests that arrive when the local cache is empty share one Redis lookup instead of each firing their own
  • Example stub webhook included under authbridge/demos/session-budget/
• Cold-cache behavior is now mode-dependent (and documented)
  • pause synchronously hydrates from Redis on the request path so pre-existing over-budget sessions fire the webhook on first request
  • deny / observe (for on_exceed) keep the pre-existing behavior: skip on cold-cache, counters populate as inference responses stream back and via the background refresh loop. Keeps Redis off the hot path for the common modes.
• Added unit and e2e tests for pause mode

Assisted-By: Claude (Anthropic AI) noreply@anthropic.com

Related issue(s)

Closes #759

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