chore(ui): ensure isTTY is correct

[Skn0tt]

Closes #37867. Ensures reporters can use process.stdin.isTTY to detect whether they can listen for SIGINT or not. Migrates HTML reporter to use it.

Sadly, it's hard to properly test this because we'd need a TTY emulator like node-pty to turn on isTTY in the subprocess. Unsure if that's worth it, so I added a comment instead.

I'm not expecting behaviour changes from the HTML reporter change, because:

• VS Code test servers are already in a child process, so isTTY is undefined
• UI Mode test servers intercept stdio, so they'll get the faked isTTY
• Watch mode doesn't intercept stdio, so its usage of isTTY will stay the same

For 3rd party reporters that already depend on process.stdin.isTTY today, we'll have a behaviour change in UI mode, but I think that's innocent. It's also the whole point of the change.

[agg23]

Minor clarity suggestions

[github-actions]

Test results for "MCP"

1 failed
❌ [chrome] › mcp/cdp.spec.ts:20 › cdp server @mcp-windows-latest

2530 passed, 108 skipped

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Merge workflow run.

[github-actions]

Test results for "tests 1"

3 flaky

⚠️ [firefox-library] › library/inspector/cli-codegen-1.spec.ts:1079 › cli codegen › should not throw csp directive violation errors `@firefox-ubuntu-22.04-node18`
⚠️ [firefox-page] › page/page-goto.spec.ts:83 › should work with Cross-Origin-Opener-Policy `@firefox-ubuntu-22.04-node18`
⚠️ [playwright-test] › ui-mode-test-attachments.spec.ts:61 › should contain binary attachment `@windows-latest-node18-1`

47196 passed, 813 skipped

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Merge workflow run.