docs(tutorials): fix MCPToolset reference links and remote server example

[Adam-D-Lewis]

Follow-up to #59 — these small fixes to the agent tutorial were inadvertently left out before that PR merged.

• Point the MCPToolset reference links at the proper API anchor (https://ai.pydantic.dev/api/mcp/#pydantic_ai.mcp.MCPToolset) instead of the generic client guide page.
• Expand the remote MCP server snippet to show the full toolsets config block, matching the rest of the tutorial.

[pmeier]

Linking to the correct objects in our or external API references is completely automated by mkdocstrings. Please just use this syntax across the whole tutorial

[Adam-D-Lewis]

Thanks @pmeier — done. All the hardcoded ai.pydantic.dev API links in the tutorial are now autoref Python refs that resolve through the inventories (pydantic_ai.mcp.MCPToolset, pydantic_ai.models.test.TestModel, etc.) instead of hand-written URLs.

One small deviation: I kept RunContext as a short code-span ref (display text RunContext, identifier pydantic_ai.tools.RunContext) rather than the full-path form, because it appears inside the RunContext[User] and RunContext.deps expressions, where the full dotted path rendered awkwardly. Happy to switch those to the full-path form too if you'd prefer consistency.

Verified locally with mkdocs build: all refs resolve and there are no cross-reference warnings. (Incidentally, the old hardcoded URLs were already 301-redirecting to pydantic.dev's new docs domain, so this clears that drift up too.)