Allow executeTool() to return null.

[markafoltz]

This happens when executeTool() is called on a missing tool, or a result value cannot be stringified.

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[beaufortfrancois]

My understanding is that your PR changes change alone don't make null reachable: every path in the tool execute steps / imperative execute steps that yields null also passes success = false, so executeTool() returned promise always rejects with UnknownError (missing tool, serialization failure, etc.).

[markafoltz]

I see that now, but:

• Blink IDL returns Promise<DOMString?>, is there a reason for the discrepancy?
• I think having completionSteps take just a single value , with null representing failure, would be simpler to understand. But perhaps we resolve Define specific errors for tool execution failures #323 first? @domfarolino

[beaufortfrancois]

• Blink IDL returns Promise<DOMString?>, is there a reason for the discrepancy?

Yep. ? is because the declarative API returns null on cross-document navigation.

[domfarolino]

If that's the only reason, and the spec doesn't fully support declarative WebMCP and its cross-origin implications, then it sounds like maybe we don't need this (until we fully spec declarative)?

[markafoltz]

I think we can drop this until we need to accommodate the declarative API; a comment in the IDL might be useful tho.

The other changes are superseded by #339.