[codex] Fix TUI large paste placeholder numbering after Ctrl+C

[canvrno-oai]

Fixes #19940.

Large-paste placeholder numbering was backed by a per-size counter, so clearing a draft with Ctrl+C left numbering state behind even though the active pending paste state was gone. This updates the composer to derive the next placeholder suffix from active pending pastes instead, which keeps simultaneous same-size pastes distinct while letting fresh drafts reuse the base label. This is also a small code cleanup: pending paste state is now the source of truth instead of maintaining a separate counter.

Credit to @Sungyoun-Kim for the issue report, root-cause notes, and fork with the proposed fix, and to @charley-oai for the earlier related #10032 proposal.

Changes:

• Remove the monotonic large-paste counter from the composer.
• Compute suffixes from currently active pending paste placeholders.
• Document large-paste placeholder behavior in the composer module docs.
• Add regression coverage for Ctrl+C clearing and deletion/reset behavior.

Testing:

• just fmt
• git diff --check

[canvrno-oai]

@codex review

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. Already looking forward to the next diff.

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[fcoury-oai]

Replicated the issue on stable Codex and was confirmed the behavior was fixed with this branch.

Code looks good, approved.