fix(deps): require mcp>=1.19.0 for in-process SDK MCP tools (#891)

## Problem

`create_sdk_mcp_server` tool handlers have returned `CallToolResult`
objects since v0.1.51 (PR #717). The `mcp` package's
`@server.call_tool()` decorator only accepts `CallToolResult` returns
from `mcp>=1.19.0`. With older versions it falls through the iterable
branch (pydantic models iterate as `(field, value)` tuples), fails
validation, and swallows the result into `_make_error_result(str(e))`.

Effect: in-process SDK MCP tools run their handler, but the model
receives a ~5KB pydantic "20 validation errors for CallToolResult" blob
with `is_error=True` instead of the actual output. Silent data loss.

This only bites users who `pip install -U claude-agent-sdk` in an
existing env — fresh installs pull latest `mcp` and work fine.
stdio/HTTP/SSE MCP servers are unaffected.

## Fix

Bump the floor: `mcp>=0.1.0` → `mcp>=1.19.0`.

## Verification

Boundary-tested with a direct `request_handlers[CallToolRequest]` repro:

| mcp version | result |
|---|---|
| 1.12.4 | ❌ `isError: True`, validation error blob |
| 1.18.0 | ❌ `isError: True`, validation error blob |
| **1.19.0** | ✅ `isError: False`, tool output returned |
| 1.27.0 | ✅ `isError: False`, tool output returned |

- `pip install -e . 'mcp==1.12.4'` → `ResolutionImpossible` (pin
enforced)
- Full `ClaudeSDKClient` e2e with `create_sdk_mcp_server`: tool result
reaches the model
- `pytest`: 755 passed, 3 skipped · `mypy`: clean · `ruff`: clean

Reported internally.

Author: qing-ant

pyproject.toml

@@ -28,7 +28,7 @@ dependencies = [
     "anyio>=4.0.0",
     "sniffio>=1.0.0",
     "typing_extensions>=4.0.0; python_version<'3.11'",
-    "mcp>=0.1.0",
+    "mcp>=1.19.0",
 ]
 
 [project.optional-dependencies]