fix(server): select an available provider for delegated tasks

[saphid]

Summary

A driver-only delegation target (target.driverKind without providerInstanceId) could fail on an unavailable inherited provider instance even when another instance of that driver was healthy: resolveTarget preferred the parent's instance whenever it was registered and orchestration-capable, and the constraint check then rejected it.

The fix requires the inherited candidate to pass the same providerConstraints availability check as any other candidate before it can win; otherwise the first constraint-free instance of the requested driver is selected. When no healthy candidate exists, resolution falls through to the parent instance id so the constraint check reports why the inherited instance cannot serve. Explicit providerInstanceId selection is unchanged and stays strict — an unavailable explicit target fails instead of silently rerouting.

Adapted from @kiiwee's earlier proposal in #11154 (closed unmerged) onto the live adapter-registry capability lookup from #11578. Credit to kiiwee for identifying the defect and the original fix shape.

Test plan

• Focused regression: vp test run apps/server/src/mcp/OrchestratorMcpService.test.ts — the new table-driven test proves healthy inheritance (parent model selection including options preserved verbatim), unavailable parent + healthy peer fallback, no-available-peer rejection, explicit-unavailable rejection without rerouting, and explicit healthy selection. The fallback case failed before the fix (Provider codex cannot run a child task: Provider instance is disabled.).
• vp test run apps/server/src/mcp/ — all 170 MCP tests pass, including the cross-provider delegation integration test.
• Targeted vp lint, vp fmt --check, and tsc --noEmit clean for the changed scope.

Independent cross-provider review: attempted codex exec -m gpt-6-astra -c model_reasoning_effort="xhigh" --sandbox read-only on the frozen diff; the reviewer account was at its usage limit (reported retry Sep 19), so the review was skipped — disclosed, not substituted.

Targets Orchestrator V2 in #2829.

Engineering: SWE-2 Max through the Devin/T3 harness.
Coordination: T3 thread 3aa91588-f393-4df8-9d47-c8c769d27899, campaign saphid/t3code-personal#298.

[macroscopeapp]

Approvability

Verdict: Not approved

Macroscope's review found this PR not approvable — The PR is a focused, well-tested fix to delegated-task provider resolution, but it changes the implicit provider-selection behavior when no instance is specified. That default behavior change warrants human review.

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