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 the repository-pinned golangci-lint v2.14.0
• Tested locally with go test -race -json ./...

The original regression fails before the fix and passes after it. The branch now normally merges main c59e8c199efbf14467211861de44afaeeb6873e3, preserving the original commit history; the two-file contribution patch is byte-identical to the previously reviewed patch.

Validation on the updated source tree: Go 1.26.8 / macOS arm64, full go test -race -json ./... passed (8,388 test/subtest pass events, no failures); repository-pinned golangci-lint 2.14.0 reports 0 issues. npm ci, UI typechecking and production builds for all four embedded MCP Apps passed using Node 26.10.0. No generated UI files or tool snapshots are committed. The unpublished merge commit's author/committer were corrected to po-et's noreply identity; its tested source tree is unchanged.

No other-platform local tests, live GitHub/provider calls, browser visual review or natural-language model prompts were run. Local results do not imply current upstream CI success or approval of the new head.

Docs

• Not needed

This corrects an existing field without changing the tool schema.

[IrynaKulakova]

Reviewed commit 4dbf51a. No actionable findings: the merged_at fallback preserves explicit merged values and is shared by text and structured output. Local lint and tests could not run because the installed Go binaries fail with "Bad CPU type in executable"; this approval is based on code inspection.