feat(gateway): negotiate platform delivery capabilities

[NeoHsu]

Important

Stacked draft: logical base docs/teams-adr-commands-proactive is PR #1489. GitHub requires an upstream PR base to exist in openabdev/openab, so this draft temporarily targets main and may show preceding stack layers. Do not merge it until #1489 is merged and this branch is rebased onto current main; then review only its single incremental commit.

What problem does this solve?

Let Core make delivery and streaming decisions from negotiated adapter capabilities instead of platform-name assumptions.

Discord Discussion URL: https://discord.com/channels/1491295327620169908/1491365158868619404/1531339032527765655
Microsoft Teams roadmap discussion.

Review Contract

Goal

Let Core make delivery and streaming decisions from negotiated adapter capabilities instead of platform-name assumptions.

Non-goals

This PR does not harden Teams HTTP transport, add route state, require write acknowledgements, or enable any new user-facing feature.

Accepted Residual Risks

Old peers still use an explicit legacy fallback and cannot provide authoritative write outcomes. Capability additions require Core-first rollout for later coupled features.

Acceptance Criteria

Hello fields remain additive and fail closed when absent; legacy LINE/LINE WORKS behavior is preserved; Unified and Standalone derive equivalent capabilities; all affected code and schema references compile and resolve.

Follow-ups

Use the negotiated contract in the later dispatch, acknowledgement, mutation, reaction, and persistent-delivery PRs.

At a Glance

Authenticated Teams activity / operator target
                    │
                    ▼
            [this PR’s bounded layer]
                    │
                    ▼
          explicit outcome or fail-closed stop

Prior Art & Industry Research

OpenClaw: its Microsoft Teams extension separates access checks, Bot Framework route context, and outbound operations. For this slice the relevant comparison is adapter capability boundaries and explicit platform behavior.

Hermes Agent: its Teams platform adapter keeps Teams-specific transport and message shaping behind a platform adapter. It does not provide OpenAB’s negotiated Core/Gateway outcome contract, so this PR keeps the useful adapter boundary but adds explicit fail-closed semantics.

Proposed Solution

• Add additive Core/Gateway hello negotiation for platform delivery capabilities.
• Model write outcomes and message limits explicitly while preserving legacy peer fallbacks.
• Refresh the LINE WORKS schema source after replacing the old non-editable platform symbol.

Why this approach?

An additive handshake keeps old peers working while removing hidden platform-name assumptions from later delivery decisions.

Alternatives Considered

Keep platform-name allowlists (rejected: they cannot describe per-adapter outcomes) or require an immediate protocol break (rejected: unsafe rolling upgrades).

Validation

• cargo check -p openab-core
• cargo check -p openab-gateway --features teams
• Platform Schema conformance on the completed stack