test(e2e): cover core session streaming, stop, and permission prompts

[chriswritescode-dev]

Summary

Adds end-to-end coverage for the core chat session flow. Before this, e2e only checked that the session page renders.

Stacked on #426: this PR uses e2e/mockProvider.ts, which #426 introduces. The base is feat/tts-visible-messages-shortcut, so the diff shows only this change. Merge #426 first; GitHub then retargets this PR to main.

New spec e2e/session-core.spec.ts

1. Send and stream: the first reply chunk is visible before the last chunk arrives, then the full reply shows and the session is idle.
2. Stop: Stop is clicked during a slow reply. The session goes idle, and the test waits past the next chunk's delivery time to confirm it never renders.
3. Permission, allow: with permission: { bash: 'ask' }, the mock model calls the shell tool. "Allow Once" runs the command (a marker file appears), and the follow-up reply shows.
4. Permission, deny: "Deny" closes the dialog. The command does not run, and the session is idle.

Mock provider (e2e/mockProvider.ts)

• Selects replies by a trigger keyword in the latest user message, not by queue order. OpenCode also sends title-generation requests to the same endpoint, so a queue would hand replies to the wrong request.
• Adds chunked streaming with a delay, OpenAI-format streamed tool_calls, a fixed follow-up after a tool result, and a clean stop when the client disconnects.
• createMockRepo accepts extra opencode.json config, and waits until OpenCode has loaded the mock model for the new directory. Without the wait, a session could start on OpenCode's default model and fail.
• setReply(text) is unchanged for the existing specs.

Validation

• Full e2e suite: 25/25 passed. session-core.spec.ts with --repeat-each=3 --workers=1: all passed.
• e2e files typecheck clean with strict tsc. They are not covered by pnpm typecheck.

Type of Change

• Bug fix
• New feature
• Refactor
• Documentation
• Tests (e2e only, no app code changes)

Checklist

• Code follows project style (no comments, named imports)
• TypeScript types are properly defined
• Tests added/updated (80% coverage target)
• pnpm lint passes locally
• pnpm typecheck passes locally

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