[Bug]: MCP tool calls emit FastMCPDeprecationWarning for inputSchema/isError/mimeType on fastmcp>=4.0

Is there an existing issue for the same bug?

• I have searched existing issues and this is not a duplicate.

Bug Description

Every MCP tool call logs FastMCPDeprecationWarning for Tool.inputSchema, CallToolResult.isError, and ImageContent.mimeType, because five call sites in the SDK still read the pre-4.0 field names (mcp/tool.py x4, mcp/definition.py x2, agent/agent.py, agent_server/mcp_router.py). openhands-sdk pins fastmcp>=3.2.0 with no upper bound, so any fresh, non-locked install can resolve fastmcp>=4.0, which renamed these fields to input_schema/is_error/mime_type and kept the old names only as deprecated compat properties.

Expected Behavior

MCP tool calls should not emit deprecation warnings on a supported, freshly-resolved fastmcp version.

Actual Behavior

Reproduced with fastmcp==4.0.1 / mcp==2.1.1 installed over the repo's own venv:

python -c "
import mcp.types as t
tool = t.Tool(name='x', inputSchema={'type': 'object'})
_ = tool.inputSchema  # triggers the warning below
"

FastMCPDeprecationWarning: Accessing `Tool.inputSchema` is deprecated; MCP SDK v2 renamed this field to `input_schema`. Update your code to read `.input_schema` instead.

The same happens for CallToolResult.isError and ImageContent.mimeType at runtime whenever an MCP tool call returns through MCPToolObservation.from_call_tool_result.

Steps to Reproduce

1. uv pip install "fastmcp==4.0.1" inside the repo's own venv (its own lockfile pins fastmcp==3.2.0, where the old names are the only ones that exist, so this simulates a consumer whose independent dependency resolution lands on a newer, still-compatible fastmcp).
2. python -m pytest tests/sdk/mcp/ -W error::FastMCPDeprecationWarning -q (or just call any MCP tool with warnings elevated to errors).
3. Warnings/errors on Tool.inputSchema, CallToolResult.isError, ImageContent.mimeType.

Acceptance Criteria

• No FastMCPDeprecationWarning on a supported fastmcp version >= the pinned floor
• Existing test suite still passes unchanged on the currently pinned fastmcp==3.2.0 (no dependency floor bump required)

Installation Method

pip install -e . (repo dev install via make build)

SDK Version

main branch, commit 7a01890

Python Version

3.13.13

Additional Context

Fix proposed in #4924: a small compat_attr(obj, new_name, old_name) helper that reads whichever name the installed mcp.types object actually has, instead of bumping the fastmcp floor (which was tried first but surfaced an unrelated MCP session-persistence regression on fastmcp==4.0.1 — worth its own issue/investigation, out of scope here).

Start with tests/sdk/mcp/ and reproduce with warnings elevated to errors using fastmcp==4.0.1. Inspect the listed call sites in mcp/tool.py, mcp/definition.py, agent/agent.py, and agent_server/mcp_router.py, along with the proposed compatibility approach in issue #4924. Done means MCP calls emit no deprecation warnings on supported versions while the existing suite passes with fastmcp==3.2.0.