#20873 base red: warm the dev server in three agent pane browser tests

Fixes a base red. check:browser-lanes (#20733) requires every browser test that starts a Vite dev server to call warmViteServer(vite, page) once it listens. Three agent pane tests landed after #20733 without that call, so web / react-apps-check is red on feat-cmux-next for every PR:

• agent-pane-session-pages (#20752)
• agent-pane-edited-calm (#20780)
• composer-row-rhythm (#20785)

This adds the call to all three.

Testing

On sbx:

• bun run check:browser-lanes:
  • red on base, which lists these files;
  • green with this change: "Browser test lane imports checked."
• CMUX_BROWSER_TESTS=1 bun test on the three files: 8 + 8 tests pass, 0 fail.
• bunx vp check is clean.

Changelog

none

🤖 Generated with Claude Code

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Summary by cubic

Fixes the failed check:browser-lanes by warming the Vite dev server in three agent pane browser tests that didn't call it.

• Adds warmViteServer(vite, "/") after vite.listen() in agent-pane-edited-calm, agent-pane-session-pages, and composer-row-rhythm tests, matching the requirement from #20733.
• Unblocks the red web / react-apps-check job on feat-cmux-next.