Add service tier overrides to spawned agents

[aibrahim-oai]

Why

Spawned agents can already override model and reasoning_effort, but they have no equivalent way to opt into a model-supported service tier. That makes it impossible to preserve or intentionally select tiered execution behavior when delegating work to a sub-agent, even though the model catalog already advertises supported service_tiers.

What changed

• Add optional service_tier to both legacy and MultiAgentV2 spawn_agent tool inputs.
• Show each picker-visible model's supported service tier ids and descriptions in the spawn_agent tool guidance.
• Resolve service tier selection after the child agent's effective model is known.
• Inherit the parent tier when omitted and still supported by the final child model; otherwise clear it.
• Reject explicit unsupported tier requests with a model-facing error.
• Keep explicit service_tier usable on full-history forks, while still honoring the existing model/reasoning fork restrictions.
• Hide service_tier alongside other spawn metadata when hide_spawn_agent_metadata is enabled.

Verification

Added focused coverage for:

• v1/v2 spawn_agent schema exposure for service_tier
• tier descriptions in spawn guidance
• hidden-metadata suppression
• explicit supported tier selection
• explicit unknown and unsupported tier rejection
• inherited tier preservation or clearing based on child-model support
• full-history fork acceptance for explicit service tiers in both v1 and v2

Local Rust tests were not run in this workspace per repo guidance; the new coverage is included for CI.