Improve telemetry output for silent tool calls

[nancyjlau]

silent successful tool calls were showing up as (No content) in the UI, from commands that succeed but don't produce any stdout (such as echo "..." > readme-copy.txt that writes to a file and doesn't print anything), adding that when a tool result is successful but empty, output "Tool executed successfully" in the output so it's clear what is happening

this also follows the existing behavior in the openai chat formatting that has the same success fallback for empty tool results, but this adds it to only to the hud UI telemetry trace tracking so it's clear what is happening (seems to be separate from the model facing openai.py whereas this is exposed to us as the user reading the traces being presnted only)

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Note

Low Risk
Low risk: change is limited to telemetry serialization of MCPToolResult to improve trace readability, with added unit coverage. Potential impact is only on how tool results are represented in spans/UI, not on tool execution.

Overview
Telemetry serialization now special-cases MCPToolResult so successful but empty tool results no longer appear as (No content); instead they get a default "Tool executed successfully" text block unless real content or structuredContent is present.

Adds focused tests to ensure the fallback is applied only for silent successes and that existing tool output is preserved.

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