Harden app-server additionalContext boundaries

[andrei-hasna]

Summary

• Reject client-supplied additionalContext kind application on turn/start and turn/steer; accepted client entries map to untrusted context.
• Enforce explicit additionalContext entry count, key length, per-value byte, and aggregate byte limits before model submission.
• Add focused app-server regression coverage and document the trust boundary and limits while preserving current source provenance metadata.

Validation

Focused validation before rebase:

• cd codex-rs && UV_CACHE_DIR=/tmp/codewith-uv-cache just fmt
• cd codex-rs && just test-fast -p codex-app-server --test all turn_start_additional_context
• cd codex-rs && just test-fast -p codex-app-server --test all turn_steer
• cd codex-rs && just test-fast -p codex-app-server-protocol
• cd codex-rs && just fix -p codex-app-server
• cd codex-rs && just fix -p codex-app-server-protocol
• git diff --cached --check
• staged credential-pattern scan: no matches
• adversarial review: no blocking findings

Rebase verification:

• Rebased to current origin/main via temporary Git index because the managed task worktree blocks checkout/rebase updates to read-only .codewith/CODEWITH.md.
• Rebased head: 992e7adb295740f5b35b892e517ee40faf36dd7d.
• GIT_INDEX_FILE=/tmp/codewith-484a5062-index git diff --cached --check origin/main
• rustfmt --edition 2024 --check on the rebased resolved Rust files
• rebased credential-pattern scan: no matches
• gh pr view: mergeable after force-push

Known CI blocker:

• cargo-deny currently fails on crossbeam-epoch 0.9.18 / RUSTSEC-2026-0204, which is present on current main and unrelated to this diff.
• Existing dedicated fix: fix(deps): bump crossbeam-epoch to 0.9.20 (RUSTSEC-2026-0204) #174 (fix(deps): bump crossbeam-epoch to 0.9.20 (RUSTSEC-2026-0204)).

Task: 484a5062-a5f6-4ef8-9465-8da89a625b6e