fix private pool fallback

[luke-lombardi]

Summary by cubic

Fixes private pool fallback to managed pools by enforcing concurrency limits and honoring pool fallback settings. Adds early capacity checks and clear failure reasons to avoid silent failures.

• Bug Fixes

  • Allow managed fallback only when Pool.Fallback is empty or internal (privatePoolAllowsManagedFallback).
  • Reserve managed quota before scheduling via managedConcurrencyLimit; fail with managed_fallback_concurrency_limit when blocked.
  • Check for an available/pending/provisionable managed path (hasManagedFallbackPath); fail with managed_fallback_no_capacity if none exists.
  • Treat any private pool as quota-exempt in privatePoolQuotaExempt; tests updated accordingly.

• Refactors

  • Extracted managedConcurrencyLimit from getConcurrencyLimit for reuse in fallback quota checks.

^{Written for commit 75952ae. Summary will update on new commits.}