Remove legacy SchedulingHintResolver after SchedulingMetadata ingestion

[eric-tramel]

Priority Level

Medium

Epic: #645
Depends on: #641, #646
Related: #644, #652
Target branch: epic/645-async-scheduling while the epic is active.

Source of Truth

Specification details live in PR #658 under plans/645/, especially migration-and-cleanup.md, contracts.md, task-admission.md, and issue-map.md. This issue tracks the cleanup slice and gates only.

Implementation Scope

Remove or collapse the legacy SchedulingHintResolver path after SchedulingMetadata ingestion is stable.

Acceptable end states are:

• the legacy resolver is deleted; or
• it is renamed/refactored into the metadata-oriented bridge from Ingest SchedulingMetadata into async scheduler grouping #646 with no independent introspection fallback.

This issue owns migration of imports, tests, and search gates for the old hint resolver names and helper functions.

Quality Gates

• AsyncTaskScheduler no longer constructs or calls the legacy SchedulingHintResolver contract.
• No duplicate production path independently derives model/provider scheduling identity outside get_scheduling_metadata() and the Ingest SchedulingMetadata into async scheduler grouping #646 bridge.
• Broad scheduler-side metadata except Exception fallback is removed in favor of typed metadata fallback.
• Existing grouping behavior remains covered for local, model, custom-model, multi-alias, partial fallback, multi-output, and former is_llm_bound cases.
• No public plugin-facing docs/API expose scheduler queue or admission internals.

Validation

Run focused scheduler metadata tests, async scheduler tests, fair queue tests as affected, make check, and repo searches for SchedulingHintResolver, SchedulingHint, _model_aliases_for_generator, independent scheduler is_llm_bound lookup, and compatibility reexports. Current production matches should be gone except intentional historical text.