#15142: fix(ui): fallbacks input always empty even when config has models (#15131)

#### Summary The Fallbacks input field on the Agent Overview page is always empty, even when `agents.defaults.model.fallbacks` has entries. The Primary Model label correctly shows `(+2 fallback)` but the input below it doesn't get populated. Closes #15131 lobster-biscuit #### Repro Steps 1. Set `agents.defaults.model.fallbacks` in `openclaw.json` with 2+ models 2. Open Control UI → Agents → Overview tab 3. The "Fallbacks (comma-separated)" input is empty despite the label showing the correct count #### Root Cause `renderAgentOverview` in `agents.ts` resolves fallback models from `config.entry?.model` only. Single-agent setups (and any agent without a per-agent model override) don't populate `config.entry.model`, so `resolveModelFallbacks` returns `null` and the input stays empty. The label works because `resolveModelLabel` already checks both entry and defaults. #### Behavior Changes - Before: Fallbacks input always empty when using global defaults - After: Fallbacks input shows the configured fallback models from `agents.defaults.model.fallbacks` #### Tests - `agents-fallbacks.node.test.ts` — covers `resolveModelFallbacks` for undefined, string, object with fallbacks/fallback, missing fallbacks, and non-string filtering - build, check pass **Sign-Off** - Models used: N/A - Submitter effort: traced the mismatch between the label and input field, one-line fallback chain fix - Agent notes: N/A <!-- greptile_comment --> <h2>Greptile Overview</h2> <h3>Greptile Summary</h3> This PR fixes the Agents → Overview “Fallbacks (comma-separated)” field not being pre-populated when fallback models are only configured via `agents.defaults.model.fallbacks`. It does this by falling back to `config.defaults?.model` when `config.entry?.model` doesn’t define fallbacks, and adds a Vitest unit test file covering `resolveModelFallbacks` behavior for several model shapes (string, object w/ `fallbacks` or `fallback`, missing, and mixed-type arrays). <h3>Confidence Score: 4/5</h3> - This PR is likely safe to merge after ensuring the new test actually runs in CI. - The functional change is small and localized (fallback chain in `renderAgentOverview`) and matches existing UI behavior for labels. The main remaining concern is whether the newly-added `*.node.test.ts` file is picked up by the project’s Vitest include pattern; if it isn’t, the regression coverage claimed by the PR won’t exist. - ui/src/ui/views/agents-fallbacks.node.test.ts (confirm Vitest includes this pattern or rename) _{Last reviewed commit: 757c4cf} <!-- greptile_other_comments_section --> **Context used:** - Context from `dashboard` - CLAUDE.md ([source](https://app.greptile.com/review/custom-context?memory=fd949e91-5c3a-4ab5-90a1-cbe184fd6ce8)) - Context from `dashboard` - AGENTS.md ([source](https://app.greptile.com/review/custom-context?memory=0d0c8278-ef8e-4d6c-ab21-f5527e322f13)) <!-- /greptile_comment -->