feat: migrate MCP tooling to mcp 2.x

[ProgrammerPlus1998]

Link to Issue or Description of Change

1. Link to an existing issue (if applicable):

• Closes: Support mcp 2.x (Python SDK): pin blocks mcp 2.0.0 #6532

Problem:

adk-python pins the MCP Python SDK below 2.0 (mcp>=1.24,<2), which blocks
installing alongside mcp 2.0.0. The 2.0 release makes several breaking changes
to the SDK surface that the MCP tool integration relies on.

Solution:

Bump the dependency to mcp>=2.0.0,<3 and adapt to the 2.0 API:

• McpError → MCPError; ProgressFnT now lives in mcp.shared.dispatcher
  (mcp.shared.session was removed).
• Read snake_case model attributes (input_schema, output_schema, is_error,
  mime_type) instead of the removed camelCase aliases.
• ClientSession(read_timeout_seconds=...) now takes float seconds rather than
  a timedelta.
• The public McpHttpClientFactory moved to a private module, so declare the
  equivalent factory Protocol locally instead of importing it.
• to_mcp_server: mcp.server.fastmcp was removed; build the server with
  mcp.server.mcpserver.MCPServer instead.

Behavior change worth a closer look (review requested):

In mcp 2.0 the server-side Context.session is a fresh ServerSession per
request rather than one stable object per connection. The agent-as-MCP-server
feature kept one ADK session per MCP connection keyed on ctx.session, which no
longer works. _connection_key now keys that map on the underlying shared
Connection object (read via ctx.session._connection, falling back to the
session itself). Connection is shared by every request on a connection and is
weak-referenceable, so WeakKeyDictionary cleanup still works. This touches a
private SDK attribute because mcp.server.mcpserver.Context exposes no public
per-connection identifier — happy to switch to a public hook if the maintainers
can point me at one.

Testing Plan

Unit Tests:

• I have added or updated unit tests for my change.
• All unit tests pass locally.

Updated the MCP tool unit tests to the 2.0 surface (snake_case attributes,
MCPError(code=..., message=...), float timeouts, and an in-memory
client/server helper replacing the removed
create_connected_server_and_client_session).

tests/unittests/tools/mcp_tool/ ......... 226 passed, 0 failed

(run against an environment with mcp==2.0.0 installed)

Manual End-to-End (E2E) Tests:

Verified _connection_key against the real mcp 2.0 SDK: two ServerSession
objects built on one shared Connection resolve to the same key, distinct from
either per-request session, and a session object without _connection falls
back to itself.

To exercise the agent-as-server path manually:

from google.adk.agents.llm_agent import LlmAgent
from google.adk.tools.mcp_tool._agent_to_mcp import to_mcp_server

agent = LlmAgent(name="assistant", model="gemini-2.0-flash", ...)
server = to_mcp_server(agent)
server.run(transport="stdio")

then call the exposed tool twice over one MCP connection and confirm the second
call continues the same ADK conversation.

Checklist

• I have read the CONTRIBUTING.md document.
• I have performed a self-review of my own code.
• I have commented my code, particularly in hard-to-understand areas.
• I have added tests that prove my fix is effective or that my feature works.
• New and existing unit tests pass locally with my changes.
• I have manually tested my changes end-to-end.
• Any dependent changes have been merged and published in downstream modules.

Additional context

The dependency-floor changes in mcp 2.0 (pydantic>=2.12, anyio>=4.9,
typing-extensions>=4.13, plus new required opentelemetry-api and
mcp-types) are satisfied by ADK's existing constraints.

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[ProgrammerPlus1998]

I've signed the CLA. Could a maintainer please re-trigger the CLA check?