obsolete: hidden background agent auth provider

[adrian-openai]

Note

This PR is obsolete for the current simplified single-run-task Agent Identity stack. It has already been merged, has no current review surface, and should not be treated as part of the active PR sequence.

The active stack is:

• #19047 - run task identity primitives
• #19049 - ChatGPT auth can register/reuse a persisted Agent Identity record
• #19051 - first-party inference and compact requests use the run task AgentAssertion path

The text that used to be here described the earlier external-task-ref/background-helper design. That shape has been collapsed out of the stack, so reviewers should ignore this PR when reviewing the current design.