fix(delegation): honor delegation.fallback_providers for child agents

[andrexibiza]

Summary

Honor a delegation.fallback_providers chain for delegated children so a worker explicitly routed via delegation.* no longer silently escalates onto fallback models intended only for the head agent (Fixes #65038).

Why this matters to users

Before: You set delegation.provider: xai-oauth so subagents run on xAI. When the child hit a retryable error, Hermes still walked the head agent's fallback_providers list — which often put ollama-cloud first — and burned through an exhausted Ollama quota while ignoring the provider you pinned.

After: You can give workers their own fallback chain under delegation.fallback_providers. Set [] to disable child fallback entirely. If you leave the key unset/null, children keep inheriting the parent chain (backward compatible with #7481).

Changes

• tools/delegate_tool.py — _build_child_agent() resolves child chain as delegation.fallback_providers > parent inheritance. Key absent/null → inherit parent _fallback_chain; [] → no child fallback; malformed → warn + inherit (mirrors reasoning_effort). Normalization reuses hermes_cli.fallback_config.get_fallback_chain().
• hermes_cli/config_defaults.py — delegation.fallback_providers: None (null sentinel = inherit; no _config_version bump).
• cli-config.yaml.example + docs — document the key and the three-way semantic.
• Tests — TestDelegationFallbackChain (override / absent / null / empty / malformed / real config loader). Async capacity test drains blocker completions to kill a latent race.

Credit / prior art

Supersedes/reimplements the approach in open PR #65052 (@ayushnangia) against current main with DCO + green local suite. Prior PR had no reported checks; this branch is a clean rebase of that design onto 6e9cae6ac4b.

How to test

scripts/run_tests.sh tests/tools/test_delegate.py tests/tools/test_async_delegation.py

Local result: 89 passed, 0 failed.

Minimal config repro from #65038:

delegation:
  provider: xai-oauth
  model: grok-composer-2.5-fast
  fallback_providers:
    - provider: xai-oauth
      model: grok-composer-2.5-fast

Platforms

• Windows (native) + git-bash test runner via scripts/run_tests.sh

Interlock

• Fixes delegation.fallback_providers is ignored; delegated workers inherit the parent fallback chain #65038
• Related: fix(delegation): honor delegation.fallback_providers for child agents (#65038) #65052 (prior open attempt — credit preserved)
• Related: Feature request: per-delegation fallback provider chain #7481 (parent-chain inheritance, preserved when key absent)

Part of #65038

Part of #80450

[andrexibiza]

Interlock receipt

• Fixes delegation.fallback_providers is ignored; delegated workers inherit the parent fallback chain #65038 (bidirectional: issue comment links this PR)
• Related / credit: fix(delegation): honor delegation.fallback_providers for child agents (#65038) #65052 (@ayushnangia) — prior open implementation of the same design; this PR is a clean reimplementation on current main with DCO + green suite
• Preserves Feature request: per-delegation fallback provider chain #7481 parent-chain inheritance when delegation.fallback_providers is absent/null

Local: 89/89 green on tests/tools/test_delegate.py + tests/tools/test_async_delegation.py.

[alt-glitch]

This was generated by AI during triage.

Duplicate of #65052, which is the earlier open implementation of the same child fallback-chain resolution.

[andrexibiza]

ci: force re-trigger — queued run never associated; reopening immediately

[andrexibiza]

Rebased onto current origin/main and resolved the conflict that was blocking mergeability.

What changed:

• Rebased fix/delegation-fallback-providers-65038 onto origin/main (4323c67dcc6).
• Resolved two merge conflicts in tools/delegate_tool.py and cli-config.yaml.example that had arisen because main already landed the #80450 pinned-provider fallback behavior while this PR adds delegation.fallback_providers.
• Semantic merge: explicit delegation.fallback_providers (declared child chain, [] disables) now takes precedence; when unset, children inherit the parent chain subject to the #80450 exception (an explicit delegation.provider pin fails loudly rather than silently rerouting onto parent fallback models). Malformed fallback_providers entries log a warning and fall back to parent inheritance, matching the PR's own test contract.
• Kept the newer child_session_db ownership-transfer pattern from main (the PR branch predated that refactor).
• Preserved the additive doc comments from both sides in cli-config.yaml.example.

Verification: tests/tools/test_delegate.py + tests/tools/test_async_delegation.py → 98 passed; tests/hermes_cli/test_fallback_config.py → 5 passed; attribution audit clean; git diff --check clean. All commits carry andrexibiza's GitHub noreply identity.

PR is now mergeable; CI is re-running on the new head.

[andrexibiza]

@ayushnangia retains all credit for this fix. Freshly rebased.