test(desktop): wait for Side Chat send readiness between follow-ups

[liugddx]

Summary

Refs #5327. Addresses the Side Chat queue failure observed in #5316; the separate Skill Draft menu-click failure remains under investigation.

A follow-up can appear in the queue before the submit IPC response settles. Composer still disables Send and ignores Enter during that interval, so the E2E can leave successor two in the editor instead of submitting it. Wait for the existing Send button to be enabled before each consecutive follow-up. All queue, steering and reconnect assertions remain intact; there are no timeout increases or retries.

Verification

• Controlled Electron probe: held the real Host submit response after admission. The first queue row appeared while Send remained disabled; a second Enter made no Host call and retained its draft. Releasing the response and waiting for Send readiness admitted the second message. All behavioral assertions passed; the probe's Windows teardown then failed deleting the Chromium DIPS file with EBUSY, so the overall probe run was not green.
• Desktop build with workspace dependencies and renderer: passed.
• Biome check, ASF header check, git diff --check: passed. E2E inventory remains 38 tests in 22 files.
• Unmodified Windows main and feat(desktop): provide managed HTML artifact preview endpoints #5316 baselines each passed both Skill Draft cases. Side Chat passed the original hosted failure point, but failed later (overall timeout / reconnect evaluation).
• Post-change Windows run: all three selected cases failed before their test bodies at Electron firstWindow startup. Full E2E validation therefore remains pending on Linux CI.
• The Windows pre-commit wrapper could not spawn biome.cmd (EINVAL); the equivalent Biome check was run directly and passed before bypassing that wrapper for this commit.

Pending validation

• Verify the complete Side Chat E2E on hosted Linux.

AI use

• No generative tool made a substantive contribution
• Generative tooling made a substantive contribution

Tool(s) and scope: Codex investigated CI artifacts, performed the controlled reproduction, and implemented this five-line test synchronization change.

Checklist

• A controlled reproduction demonstrates the premature Enter failure and the enabled-Send synchronization
• All affected E2E suites pass (Linux CI pending; local startup limitation above)

Does this PR entail a change in behavior?

• Yes
• No product behavior change; test synchronization only

[liugddx]

Superseded by upstream #5315, merged while this investigation was in progress. Commit 99098aa already adds awaitSendReady(companion) to the same queue loops and other Side Chat sends, reusing the shared helper. The controlled delayed-admission probe independently confirmed the race, but this five-line PR is now redundant. Closing it and validating #5316 against the updated main instead. Skill Draft remains a separate investigation under #5327.