[Enh]: Support MCP protocol revision 2026-07-28 (stateless transport, server/discover, cacheable lists)

## Summary

DAB's MCP server advertises and implements `2025-06-18` as a fixed default
(`McpProtocolDefaults.DEFAULT_PROTOCOL_VERSION`). Two spec revisions have shipped since:
`2025-11-25` and `2026-07-28`. #3429 analysed the gap to `2025-11-25` and was closed without
a follow-up item, and there is currently no issue tracking `2026-07-28` at all.

This issue asks for a decision and, if applicable, a roadmap entry — not necessarily an
immediate implementation.

## Why this revision is different

`2026-07-28` is not additive; it restructures the transport and lifecycle:

- Protocol-level sessions and the `Mcp-Session-Id` header are **removed**.
- The `initialize` / `notifications/initialized` handshake is **removed**. Every request
carries its protocol version and client capabilities in `_meta`.
- A new **`server/discover`** RPC is mandatory for advertising versions, capabilities and identity.
- The GET stream endpoint and `resources/subscribe` are replaced by **`subscriptions/listen`**.
- SSE resumability (`Last-Event-ID`) is **removed**.
- `MCP-Protocol-Version`, `Mcp-Method` and `Mcp-Name` headers become **required**, with
server-side header/body validation (`-32020 HeaderMismatch`).
- All results carry a `resultType`; server-initiated requests are replaced by the
Multi Round-Trip Requests (MRTR) pattern.
- `ping`, `logging/setLevel` are removed; Roots, Sampling and Logging are deprecated.

Because the handshake itself is gone, the compatibility burden shifts entirely to clients.
The spec defines a probe-and-fall-back path, but a server that only speaks `2025-06-18`
depends on every client retaining that path indefinitely. #3429 documented this failure mode
in practice (a client disconnecting on version downgrade over stdio).

## What DAB would gain beyond conformance

Several of the changes map onto capabilities DAB users already ask for:

- **Statelessness.** With no protocol session, DAB instances become trivially scalable behind
a load balancer, and a restart no longer invalidates in-flight clients. It also removes the
`Mcp-Session-Id` friction that the MCP Inspector guidance currently works around.
- **`ttlMs` / `cacheScope` on `tools/list`, `resources/list`, `resources/read`.** DAB already
has an entity cache with TTL; surfacing it at the protocol level would let clients stop
re-fetching. This matters at scale: our production configuration has 315 entities and
`describe_entities` responses are substantial.
- **Deterministic tool ordering** (a SHOULD in this revision) improves client-side caching and
LLM prompt-cache hit rates. Should be trivial, since DAB generates the tool surface.
- **`Mcp-Name` header.** Lets reverse proxies route, rate-limit and audit per tool without
parsing the request body. Today an operator behind nginx sees only `POST /mcp` and cannot
tell which entity was read — a real gap for auditing.
- **Tasks extension** (`io.modelcontextprotocol/tasks`) is a natural fit for
`aggregate_records` over large tables.

## Deployment context

Read-only ERP database exposed through DAB 2.0.9, HTTP transport behind an nginx reverse
proxy, 315 entities, consumed by MCP clients. Nothing is broken today — clients still
negotiate `2025-06-18` successfully over HTTP. The concern is that continued operation
depends on client-side backwards compatibility that operators do not control, while DAB has
no stated position.

## Ask

1. Is support for `2026-07-28` on the roadmap?
2. If yes, could it be tracked with a milestone/label the way other MCP work is (`2.x`, `2.3`)?
3. If no — please say so explicitly. A clear "DAB stays on `2025-06-18` for now" is genuinely
actionable for operators, who can then plan a translating layer instead of waiting.
(Related: #3658, packaging `Microsoft.DataApiBuilder.Mcp` as a NuGet SDK, would make such
a layer considerably cheaper to build.)

## References

- Changelog for the revision: https://modelcontextprotocol.io/specification/2026-07-28/changelog
- Streamable HTTP transport: https://modelcontextprotocol.io/specification/2026-07-28/basic/transports/streamable-http
- #3429 — gap analysis for `2025-11-25` (closed)
- #3658 — `Microsoft.DataApiBuilder.Mcp` NuGet package

MCP 2026-07-28仕様と#3429のギャップ分析から始め、関連する#3658のpackaging issueも確認してください。このissueでは実装ではなく、ロードマップまたは明示的な決定を求めています。そのため、該当するmilestoneまたはlabelとともにDABのサポート方針が記録されれば完了です。

索引モデルが issue の本文から書いたものです。