e2e: cover emit() content blocks on self-host

[RhysSullivan]

Fixes #1592

A self-hosted user reported that calling emit() inside execute counts the
emitted items in the result envelope, but no content actually reaches the MCP
client — only console logs and the return value came through.

I added a self-host e2e scenario that emits a plain string, a ToolFile, and a
structured object, then asserts on the actual JSON-RPC tool result's
content array (not just the emitted count). Ran it narrowly against a
real self-host MCP server and it passes: all three emitted items show up as
content blocks, matching the same code path cloud already exercises.

Since the scenario passes on main, this looks client-side rather than a
server regression — the PR is the added coverage as evidence, not a server
change.

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Torn down — the PR is closed.

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commit: de81e19