fix windows paste behavior and add more test

[occurrent]

A regression slipped in after #8021, and our existing tests did not catch it. This PR fixes that regression, tightens the PasteBurst / ChatComposer heuristics around Windows key-event pastes, and adds targeted regression tests to prevent similar breakages.

changes

• Improve classification of Windows "key-event pastes", including slower initial inter-key gaps that previously caused pastes to be split or treated as normal typing.
• Preserve ordering for multiline pastes when the first ASCII char is temporarily held (pending_first_char) and an early Enter arrives, avoiding mid-stream submit or dropped newlines.
• Fix burst timeout accounting when Enter promotes a pending-first-char into an active burst, preventing premature flush and paste chunking.
• Narrowly scope any widened Enter/newline decision window to the single-ASCII-char (hold-capable) path, to avoid IME / non-ASCII behavior regressions.

User-visible impact

• Windows: multiline key-event pastes are less likely to be submitted accidentally; Enter is more consistently treated as newline within the paste.
• Non-Windows (and environments where bracketed paste works): no intended behavior change.

Tests and docs

• Add Windows-gated regression tests covering Char -> Enter slow-gap classification, ordering, Enter/newline boundaries, and large paste placeholder paths (no stray prefix chars).
• Update docs/tui-chat-composer.md and relevant docstrings to reflect the refined heuristics and rationale.

[chatgpt-codex-connector]

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[occurrent]

@etraut-openai This PR has been up for a while, could take a look?

[etraut-openai]

@occurrent, sorry it has taken so long to get to this. The Codex team has been incredibly busy lately preparing for the recent Codex App launch. I'm still interested in this PR if you're willing to give us a bit more time.

[occurrent]

@codex review

[chatgpt-codex-connector]

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[occurrent]

@codex review

[chatgpt-codex-connector]

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Reviewed commit: ba1b456cb1

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[occurrent]

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[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. What shall we delve into next?

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[occurrent]

I have manually tested the compiled program on the Windows platform, and so far, the scenarios I designed are behaving as expected.