stacklok / toolhive / 18968926888

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Handle partial flushes in tool config middleware (#2404)

Tool config middleware implicitly assumed that `Flush()` was only
called once a full SSE event was received by the proxy. While testing
remote servers we encountered a scenario in which a very long
`tools/list` (~80kb) caused golang reverse proxy implementation to run
`Flush()` more than once on partial SSE payloads.

Golang standard library code that triggered the bug is here
https://github.com/golang/go/blob/go1.25.0/src/net/http/httputil/reverseproxy.go#L708-L711

This change checks whether the SSE event is fully buffered, and keeps
buffering otherwise. It also ships an `testkit` enhancement that lets
the test developer wrap the test MCP server in a reverse
proxy. Ideally, we would use the same reverse proxy used by ToolHive
itself, but it's tightly coupled with container orchestrator and
requires refactoring.

Fixes #2403

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