fix(shared,delegate-task,claude-code-agent-loader): guard model parsers against non-string input (fixes #4145)

After the 4.2.0 unified-dispatch refactor (a42f894f / df198d8b / fee515c5 / 989ab717 / dd3fecaf / 1bbe065c / 12bd6580), at least one caller in the new prompt-async-gate path forwards a FallbackModelObject (or some other non-string shape) into parsers that statically claim 'model: string'. The downstream .trim() call then throws 'model.trim is not a function', which rejects the session.processor promise and surfaces as 'Aborted process' + UI 'interrupted'. The issue (#4145) reports this aborts 90% of subagent dispatches across every provider on 4.2.0 + opencode 1.15.4.

This patch adds a 'typeof x !== "string"' runtime guard at the four parser entrypoints called from the dispatch path:

- src/shared/fallback-chain-from-models.ts :: parseVariantFromModel, parseFallbackModelEntry

- src/tools/delegate-task/model-string-parser.ts :: parseVariantFromModelID, parseModelString

- src/shared/model-string-parser.ts (duplicate file with same API) :: parseVariantFromModelID, parseModelString

- src/features/claude-code-agent-loader/claude-model-mapper.ts :: mapClaudeModelString

Each parser now returns undefined / { modelID: "" } for non-string input instead of throwing. This unblocks subagent dispatch and leaves the underlying caller bug for a follow-up.

Regression coverage: three new tests in src/shared/fallback-chain-from-models.test.ts pin the non-string behavior (object, null/undefined, number). Existing 38 tests still pass. Total: 41/41 green, typecheck clean.

src/features/claude-code-agent-loader/claude-model-mapper.ts

@@ -11,6 +11,7 @@ const CLAUDE_CODE_ALIAS_MAP = new Map<string, string>([
 
 function mapClaudeModelString(model: string | undefined): string | undefined {
   if (!model) return undefined
+  if (typeof model !== "string") return undefined
 
   const trimmed = model.trim()
   if (trimmed.length === 0) return undefined