core: pass step environments to turn input extensions

[sayan-oai]

Why

Turn-input extensions currently read environments from TurnContext, which is frozen when the turn starts. With deferred executors, run_turn may already have a newer environment snapshot for the first sampling request.

What

• pass the first request-owned StepContext through the existing skills/plugins input builder
• derive static turn data from step_context.turn
• build extension TurnInputContext.environments from the step snapshot
• leave the extension API unchanged
• preserve flag-off behavior because the first step then mirrors TurnContext

Turn-input contributors still run once for submitted turn input. Making their output reactive to environments that attach after the first request remains a separate WorldState/service-refresh change.

Tests

• just test -p codex-core turn_input_contributors_receive_step_environments
• just test -p codex-core 'session::turn::tests'
• just fix -p codex-core
• just fmt