test(delegation): executable class map for fallback-chain resolution (#80450)

[ayushnangia]

Summary

Test-only companion to the cross-PR map on #80450: an executable definition of the delegation/fallback resolution class — "which fallback chain does a non-head agent get?" No production code changes; nothing here competes with the fixes in flight.

Changes

tests/tools/test_delegation_fallback_class.py, one test per open member:

• pinned delegation.provider must not inherit the parent chain — xfail, flips to XPASS when fix(delegation): honor pinned delegation.provider — no silent parent-fallback substitution #80465 lands
• configured delegation.fallback_providers must reach the child — xfail, flips when fix(delegation): honor worker fallback providers #80438 (@wz-heng) / fix(delegation): honor delegation.fallback_providers for child agents #80421 (@andrexibiza) land (delegation.fallback_providers is ignored; delegated workers inherit the parent fallback chain #65038, reported by @mlahatte)
• explicit empty delegation chain disables inheritance — xfail, contract per fix(delegation): honor worker fallback providers #80438
• pin + declared chain composition — skip with the open precedence question from the delegate_task: pinned delegation.provider/model silently replaced at runtime by inherited parent fallback chain #80450 discussion; unskip once decided
• baseline: unpinned/unconfigured child inherits the parent chain — green, guards today's intended behavior

All xfails are strict=False: the suite is green on main today, and each member's landing turns its test into a visible XPASS — so the audit item on tracker #79686 has a checklist that runs in CI rather than in prose. Chain-entry members (#80209 explicit api_mode, #79840 benched credentials) and auxiliary-agent members (#79750) carry suites in their own PRs and are referenced in the module docstring rather than duplicated.

Validation

scripts/run_tests.sh tests/tools/test_delegation_fallback_class.py: 1 passed, 3 xfailed, 1 skipped, 0 failed. Neighbor suite tests/tools/test_delegate.py: 63/63. Harness reuses _make_mock_parent + patched run_agent.AIAgent from the existing delegate tests.

Scope notes

• The xfail reasons cite the member PRs; whichever of the fix(delegation): honor worker fallback providers #80438/fix(delegation): honor delegation.fallback_providers for child agents #80421 pair lands, the same two tests flip.
• If the precedence decision goes the other way from the suggestion in delegate_task: pinned delegation.provider/model silently replaced at runtime by inherited parent fallback chain #80450, the skipped test's assertion is a two-line edit.

[ayushnangia]

Companion map: #80450 (comment)