fix: custom prompt expansion with large pastes

[Priya-753]

Summary of Changes

What?
Fix for slash commands (e.g., /prompts:code-review) not being recognized when large content (>3000 chars) is pasted.
Bug Report
Why?
With large pastes, slash commands were ignored, so custom prompts weren't expanded and were submitted as literal text.

How?
Refactored the early return block in handle_key_event_without_popup (lines 957-968).
Instead of returning early after replacing placeholders, the code now replaces placeholders in the textarea and continues to the normal submission flow.
This reuses the existing slash command detection and custom prompt expansion logic (lines 981-1047), avoiding duplication.

Changes:
Modified codex-rs/tui/src/bottom_pane/chat_composer.rs: refactored early return block to continue to normal flow instead of returning immediately
Added test: custom_prompt_with_large_paste_expands_correctly to verify the fix

Code Quality:
No lint warnings
Code follows existing patterns and reuses existing logic
Atomic change focused on the bug fix