Always enter Toolset context when running agent

[strawgate]

• Fixes Tool Manager build() does not enter toolset context #2337
• Fixes Parallel MCP tool calls cause Runtime Error #2355
• Related to Dynamic toolsets based on run context #2331

Always enter the Agent's context in .iter -- this allows us to remove the need for the MCP Toolset to enter its context in get_tools -- which I believe was resulting in startup/shutdown for every get_tools call.

Using async with for MCP is still nice to share an instance of the mcp server across agents runs

[DouweM]

@strawgate Thanks William!

[strawgate]

It would be beneficial for my dynamic toolset pr if the agent always entered the toolset context instead of only the first time the context is activated and then we document that the toolset contexts must be reentrant / idempotent as discussed in the #2337

[strawgate]

I really thought you could pop() context managers off an ExitStack but you can't, so any contexts accumulated into an exitstack can only be exited all at once.

So I think instead of entering the agent in .iter() we should just enter the toolset in .iter() (we've now come full circle 😅).

This results in the same behavior where it matters for this but is sad because entering the Agent's context felt like a pretty clean way to handle it.

Also sorry for the force push...

[DouweM]

@strawgate Thanks William!