Fix nil update dereference in tool approval middleware

[PratikDhanave]

Summary

The tool approval middleware (agent/harness/toolapproval) dereferences every update from the inner run without a nil check: the main loop passes each update straight into splitApprovalRequestContents, which iterates u.Contents.

Nil updates are legal in a response stream — the agent run loop skips them (agent.go), and the toolautocall middleware explicitly forwards them downstream (autocall.go). toolapproval is the only consumer in the codebase missing this guard, so composing it with toolautocall (or any provider that yields a nil update) panics:

panic: runtime error: invalid memory address or nil pointer dereference
  toolapproval.splitApprovalRequestContents(0x0)
      agent/harness/toolapproval/toolapproval.go:422
  toolapproval.run.func1-range1(...)
      agent/harness/toolapproval/toolapproval.go:139

Fix

Forward nil updates downstream (matching toolautocall's behavior) instead of classifying them, preserving stream semantics for outer consumers.

No public API changes.

Tests

• Added TestToolApproval_NilUpdatePassthrough, which drives the middleware with an inner run that yields a nil update followed by a text update. On main it fails with the SIGSEGV above; with the fix it passes and asserts the nil update is forwarded and the text update is delivered.
• go test -race -shuffle=on ./... passes.
• gofmt/gofumpt clean, go vet clean.

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[Copilot]

Pull request overview

This PR fixes a panic in the tool approval middleware (agent/harness/toolapproval) when the inner run yields a nil update by forwarding nil updates downstream instead of attempting to classify them, aligning behavior with other middleware and the core run loop.

Changes:

• Add a nil-update guard in the tool approval middleware’s streaming loop and forward nil updates downstream.
• Add a regression test to ensure nil updates are tolerated and passed through.
• Apply gofmt/gofumpt-style formatting to existing test callsites.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
agent/harness/toolapproval/toolapproval.go	Forwards nil updates downstream to avoid dereferencing u.Contents on a nil update.
agent/harness/toolapproval/toolapproval_test.go	Adds a regression test for nil update passthrough and reformats helper callsites.

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