fix(review): propagate configured curator fallback chains

[Cloudymap1e]

Summary

• resolve auxiliary-agent fallback chains as task-specific entries followed by global entries
• deduplicate normalized routes while ignoring empty or malformed entries
• pass the effective chain to curator and background-review forks
• preserve the live parent fallback chain for routed and unrouted background-review forks when configuration supplies no chain
• preserve routed-runtime and cache-parity behavior

Fixes #78371

Testing

• fallback configuration, curator, background-review cost-control, and cache-parity suites — 48 passed
• Ruff on touched files — passed
• git diff --check — passed

Checklist

• Task chain precedes global chain
• Routed and unrouted forks preserve parent fallback
• Malformed/empty entries fail safely
• No credentials logged or persisted

[Halldrix]

Hi @Cloudymap1e — I independently opened #93664 for the same bug before
noticing this PR (my fault for missing it in the duplicate search); the
triage bot correctly flagged mine, and I'll be closing it. Your design
matches what I landed independently: task chain first, global chain as
safety net, backend-identity dedupe, fresh dicts, wiring into both forks.

Three things from building the same fix that may be useful here:

1. background_review ignores auxiliary fallback_chain config when primary provider fails #93592 is also fixed by this PR but won't auto-close. The
   background-review fork is wired here, yet the body only cites
   Fixes #78371. Adding Fixes #93592 would close the newer report too
   when this merges.

2. transport -> api_mode aliasing. Chain entries may declare the
   wire format as transport (_resolve_fallback_entry in
   auxiliary_client.py accepts api_mode or transport), but
   _try_activate_fallback() only reads api_mode — a transport:-only
   entry silently drops to chat_completions on the conversation-loop path.
   One normalization line in get_auxiliary_fallback_chain closes it.

3. Parent-chain inheritance is a behavior change worth surfacing.
   Keeping agent._fallback_chain when no config supplies a chain means
   previously-unconfigured installs will start failing reviews over across
   providers (full snapshot replay, cold cache). It may well be intended —
   but it goes beyond what either linked issue asks for, so a line in the
   PR body would help reviewers.