[WRONG BRANCH] fix: pin combo response child routes to concrete targets to avoid alias shadowing

[luvs01]

Motivation

• Public combo aliases could be re-resolved for already-selected concrete combo targets, allowing a shadowing alias to redirect child requests to an unintended provider/model and leak prompt payloads.
• routeModelInternal already had a bypass for concrete targets, but the combo child dispatch path still used the alias-aware public routeModel() and thus re-entered alias resolution.

Description

• Add routeConcreteModel(config, modelId) in src/router.ts that calls the internal resolver with bypassCombos=true to resolve a concrete provider/model without consulting public combo aliases.
• Use routeConcreteModel() in src/server/responses/core.ts for combo child checks and when building the child request (canDecryptUnreadableAgentTask, targetRoute, and child-request routing), ensuring child payloads remain pinned to the originally selected physical target.
• Make handleResponses call the concrete resolver for recursive combo child attempts by checking options.comboAttempt so alias-aware routing is disabled for those recursive invocations.
• Add an end-to-end regression test in tests/server-combo-failover-e2e.test.ts that verifies a shadowing combo alias cannot redirect a child request away from the selected concrete provider.

Testing

• Ran bun run typecheck (via PATH="$PWD/node_modules/.bin:$PATH" bun run typecheck) and it succeeded.
• Ran the targeted test: ./node_modules/.bin/bun test tests/server-combo-failover-e2e.test.ts --filter "dispatches a selected concrete target" and it passed.
• Ran bun run privacy:scan and it passed.
• Attempted the full bun run test; the changes are localized and the focused combo failover suite passed, but the full-suite run encountered unrelated environment-specific failures/timeouts in this container (these are unrelated to the combo routing change).

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📥 Commits

Reviewing files that changed from the base of the PR and between 726e1c7 and cda3f76.

📒 Files selected for processing (3)

• src/router.ts
• src/server/responses/core.ts
• tests/server-combo-failover-e2e.test.ts

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⏳ DRAFT

• wrong target branch (main); retarget to dev.

What to do

• Retarget this PR to dev — all contributions go to dev.

Its title has been prefixed with [WRONG BRANCH].
This pull request was already a draft. Its draft status will be preserved after every issue above is resolved.