[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

Bắt đầu với đặc tả MCP ngày 2026-07-28 và phân tích khoảng cách trong #3429; đồng thời xem xét issue packaging liên quan, #3658. Issue này yêu cầu một roadmap hoặc quyết định rõ ràng thay vì triển khai, vì vậy sẽ hoàn tất khi quan điểm hỗ trợ của DAB được ghi nhận cùng với mọi milestone hoặc label áp dụng được.

Do mô hình lập chỉ mục viết ra từ nội dung của issue.