feat: expose session context to MCP tool handlers via get_tool_context()

[Nik-Reddy]

Fixes #823

I've been building tool handlers that need to know things about the current session — like reading back conversation history to avoid repeating work, or just knowing the session ID for logging purposes. Right now there's no way to get that info from inside an @tool function.

This adds a ToolContext dataclass and a get_tool_context() function that tool handlers can call to grab session metadata (session_id, transcript_path, cwd, etc.) and optionally read back conversation history.

How it works

The SDK already receives session metadata through hook callbacks (PreToolUse, PostToolUse) before a tools/call request comes in. This PR captures that metadata into a contextvar, so by the time the tool handler runs, the context is already there.

From inside a tool handler:

`python
from claude_agent_sdk import get_tool_context

@tool
def my_tool(args):
ctx = get_tool_context()
if ctx:
history = ctx.get_conversation_history()
# do something with prior messages
`

get_conversation_history() is an explicit method (not a property) since it does file I/O to parse the JSONL transcript — wanted to make that cost obvious to the caller.

What's changed

• types.py — new ToolContext dataclass with get_conversation_history()
• _internal/_tool_context.py — contextvar module for storing/retrieving context
• _internal/query.py — captures session metadata from hook inputs, sets the contextvar before MCP tool dispatch, resets it after
• __init__.py — exports get_tool_context and ToolContext
• tests/test_tool_context.py — 16 tests covering the happy path, missing context, file I/O edge cases, contextvar isolation

Design notes

• This is a best-effort API — get_tool_context() returns None if no hooks fired before the tool call. Felt like the right tradeoff vs. raising an error.
• No parameter injection into tool signatures — tools opt in by calling the function explicitly. Keeps backward compatibility and avoids any magic.
• Conversation history comes from the existing JSONL transcript, no new storage mechanism.

All existing tests still pass (460 total).