Redesign messaging adapter base with caller-resolves pattern

What does this MR do and why?

Establishes the "caller resolves, adapter delivers" pattern for the messaging adapter base class. Callers build a TriggerBundle with fully resolved params, and adapters handle only delivery and lifecycle — no policy, no resolution.

This is stacked on !236476 (merged) which refactors ExecuteWorkflowService to accept pre-resolved flow params.

Key changes

• Adapters::Base — rewritten. trigger(bundle) accepts a TriggerBundle value object. Tiered resilience (must {} / safe {}), composite identity linking, SA membership, callback context enrichment. Abstract methods use Gitlab::AbstractMethodError.
• CallbackWorker — descendants-based adapter registry replaces hardcoded hash. Per-hook safe {} isolation. on_flow_failed passes workflow: kwarg.
• TriggerFlowService — deleted. No production callers (only consumer was unmerged !232441 (closed)).

No concrete adapters yet, no new production code paths, no feature flag needed.

References

• Stacked on: !236476 (merged)
• Design: #599691 (closed)
• ADR 008: gitlab-com/content-sites/handbook!19714
• Previous stack (superseded): !235308 (closed), !235310 (closed), !235039 (closed)
• Flow versioning: !235204 (merged)
• Epic: #590434

How to set up and validate locally

This MR adds no new production code paths. Validate via specs:

bundle exec rspec \
  ee/spec/services/ai/messaging/adapters/base_spec.rb \
  ee/spec/workers/ai/messaging/callback_worker_spec.rb

MR acceptance checklist

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