Add McpClient: Effect-native MCP client

[polRk]

Summary

Ports the Effect-native McpClient (Model Context Protocol client) from
effect-smol#2534 into Effect-TS/effect, and bases it on the MCP
protocol-version-negotiation / spec-compliance work from effect-smol#2601 so
the client relies on current, compliant schemas and a compliant server
counterpart.

Because GitHub cannot transfer PRs between repositories, this is the replacement
PR for the effect-smol → effect migration:

• Client — replaces effect-smol#2534 (@polRk)
• Base — incorporates effect-smol#2601 (@IMax153)

What was taken, how, and why

Base — effect-smol#2601 (@IMax153), 18 commits, brought in via git cherry-pick

The 18 commits of #2601 are included verbatim with original authorship
preserved because the client depends on the schema and negotiation work they
contain. The effect-smol base at #2601's branch point was byte-identical to
effect/main for these files, so the cherry-pick applied with zero
conflicts. #2601 provides:

• MCP protocol versions 2025-11-25 and 2025-06-18 with per-session
  negotiation and lifecycle enforcement.
• Fixes to the MCP schemas the client relies on — notably CreateMessageResult
  (adds required role/content), sampling metadata made optional,
  elicitation content constrained to primitives, base64 binary content, _meta.
• A Streamable-HTTP-compliant McpServer.layerHttp (session identity,
  MCP-Protocol-Version enforcement, 400/404/202, notification gating) — the
  compliant server this client is tested against.

Client — effect-smol#2534 (@polRk) + spec-compliance fixes on top

McpClient: automatic initialize/initialized handshake, stdio and Streamable
HTTP transports, typed methods for tools/resources/prompts/completion, and
host-side handlers for server-initiated sampling/roots/elicitation.

Adapted to #2601's stricter schemas, then fixed against the MCP 2025-06-18
spec and the official @modelcontextprotocol/sdk (via a conformance audit):

• Send the required Accept: application/json, text/event-stream header on every
  Streamable HTTP request (compliant servers otherwise reply 406).
• Derive supported/latest protocol versions from the shared
  McpSchema.ProtocolVersion; offer the latest and validate the negotiated one.
• Send MCP-Protocol-Version from the negotiated initialize result body
  (a compliant server is not required to echo it as a response header).
• Fork the roots/list_changed forwarder only after the handshake completes
  (MCP lifecycle ordering).
• Spawn the stdio server with extendEnv: true (a custom env no longer strips
  PATH/HOME) and drain the child's stderr (avoids deadlock on a full pipe).

Known limitations / follow-ups (tracked separately)

Deferred to keep this PR reviewable; each needs its own tests:

• SSE responses — the HTTP transport decodes JSON responses only, not
  text/event-stream. Needed for servers (including the official SDK) that
  stream POST responses.
• HTTP hang-safety — in-flight requests aren't failed on child-process exit;
  send() lacks the terminal/empty-response guards from
  RpcClient.makeProtocolHttp.
• Cancellation — interrupting a request doesn't emit
  notifications/cancelled, and inbound notifications/cancelled is a no-op.
• HTTP session lifecycle — no DELETE termination, no 404 session-expiry
  handling.
• Systemic (shared effect-rpc JSON-RPC serialization, affects McpServer
  too) — JSON-RPC notifications are emitted carrying an id, which is not
  standard-JSON-RPC-compliant for third-party peers. Out of scope here.

Test plan

• pnpm check — clean
• pnpm test packages/effect/test/unstable/ai/ — 431 passed (15 files),
  including feat: multiple span processors and metric readers #2601's schema/server/connection conformance suites and the
  McpClient stdio + Streamable HTTP suites
• pnpm lint and pnpm docgen — clean

[changeset-bot]

🦋 Changeset detected

Latest commit: 3746db9

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 27 packages

Name	Type
effect	Major
@effect/opentelemetry	Major
@effect/platform-browser	Major
@effect/platform-bun	Major
@effect/platform-node-shared	Major
@effect/platform-node	Major
@effect/vitest	Major
@effect/ai-anthropic	Major
@effect/ai-openai-compat	Major
@effect/ai-openai	Major
@effect/ai-openrouter	Major
@effect/atom-react	Major
@effect/atom-solid	Major
@effect/atom-vue	Major
@effect/sql-clickhouse	Major
@effect/sql-d1	Major
@effect/sql-libsql	Major
@effect/sql-mssql	Major
@effect/sql-mysql2	Major
@effect/sql-pg	Major
@effect/sql-pglite	Major
@effect/sql-sqlite-bun	Major
@effect/sql-sqlite-do	Major
@effect/sql-sqlite-node	Major
@effect/sql-sqlite-react-native	Major
@effect/sql-sqlite-wasm	Major
@effect/openapi-generator	Major

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