Enforce user input length cap

[etraut-openai]

Currently there is no bound on the length of a user message submitted in the TUI or through the app server interface. That means users can paste many megabytes of text, which can lead to bad performance, hangs, and crashes. In extreme cases, it can lead to a kernel panic.

This PR limits the length of a user input to 2**20 (about 1M) characters. This value was chosen because it fills the entire context window on the latest models, so accepting longer inputs wouldn't make sense anyway.

Summary

• add a shared MAX_USER_INPUT_TEXT_CHARS constant in codex-protocol and surface it in TUI and app server code
• block oversized submissions in the TUI submit flow and emit error history cells when validation fails
• reject heavy app-server requests with JSON-RPC -32602 and structured input_too_large data, plus document the behavior

Testing

• ran the IDE extension with this change and verified that when I attempt to paste a user message that's several MB long, it correctly reports an error instead of crashing or making my computer hot.

[etraut-openai]

@codex review

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. 🎉

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[owenlin0]

small comments but lgtm