Fix merged state in pull request list output

[po-et]

Summary

Correct list_pull_requests output for merged pull requests when the REST list endpoint omits the merged boolean. Use a nonzero merged_at as a fallback while preserving an explicit boolean.

Why

An agent listing closed PRs currently sees merged: false even for merged changes, which can corrupt contribution summaries. The regression reproduces this from raw REST JSON at current main.

Fixes #3467

What changed

• Fall back to the merge timestamp only when the API omits merged.
• Cover merged, closed unmerged, open, explicit true/false, and zero timestamp entries, with and without field selection.
• Verify legacy text output and modern MCP structured output.

MCP impact

• Tool schema or behavior changed

The existing merged output now correctly describes list entries. No input or output fields are added.

Prompts tested (tool changes only)

Regression calls use list_pull_requests with owner, repo, and state: "all", both with all fields and with fields: ["number", "merged", "merged_at"]. No natural-language model prompts were run.

Security / limits

• No security or limits impact

The conversion uses data already returned by the API and adds no requests.

Tool renaming

• I am not renaming tools as part of this PR

Lint & tests

• Linted locally with ./script/lint
• Tested locally with ./script/test

The regression fails before the fix and passes after it. Full tests run with the race detector and UPDATE_TOOLSNAPS=true; no tool snapshots change.

Docs

• Not needed

This corrects an existing field without changing the tool schema.