fix resume --last prompt handling and reinstate session-id conflict check

[dkhokhlov]

Addressed:

• codex exec resume --last rejects prompt when --json is set #6717 (comment)
• codex-exec: allow resume --last to read prompt #6717 #6719 (comment)

Changes

• change --last flag to accept optional prompt value (e.g. --last <PROMPT>)
• simplify prompt handling by using last_prompt() helper method
• reject invalid or nonexistent session IDs with error message instead of silently creating new conversation
• reinstate clap's conflicts_with between --last and explicit session id
• add tests for conflict validation, prompt forwarding, and invalid session ID rejection

Problems

1. Interactive codex resume --last didn't accept prompt values (#6717)

Problem: The interactive codex resume --last command used a boolean flag that couldn't accept a prompt value. Users couldn't provide a prompt directly after --last.

Before:

$ codex resume --last "next steps"
# --last was a bool flag, prompt was ignored

After:

$ codex resume --last "next steps"
# Now correctly accepts and forwards the prompt

2. codex exec resume silently ignored session_id when used with --last (#6719 discussion)

Problem: When both a session ID and --last were provided, the code silently ignored the session ID and used --last, potentially resuming the wrong session. This was unsafe for scripts and users.

Before:

$ codex exec resume 123e4567-e89b-12d3-a456-426614174000 --last
# Silently ignored the session_id and resumed most recent session
# Could touch the wrong session unintentionally

After:

$ codex exec resume 123e4567-e89b-12d3-a456-426614174000 --last
error: the argument '--last' cannot be used with '<SESSION_ID>'
# Fails fast with clear error message

3. Invalid session IDs silently created new conversations

Problem: When an invalid or nonexistent session ID was provided, codex exec resume silently created a new conversation instead of erroring, causing confusion.

Before:

$ codex exec resume sdfsdfgfds2546456 "2+2"
# Silently creates a new conversation (confusing!)
$ codex exec resume 123e4567-e89b-12d3-a456-426614174000 "2+2"
# (UUID format valid but session doesn't exist)
# Silently creates a new conversation

After:

$ codex exec resume sdfsdfgfds2546456 "2+2"
No saved session found with ID sdfsdfgfds2546456.
# Exits with error code 1
$ codex exec resume 123e4567-e89b-12d3-a456-426614174000 "2+2"
No saved session found with ID 123e4567-e89b-12d3-a456-426614174000.
# Exits with error code 1

Testing

Added tests for:

• Invalid session ID rejection (non-UUID format)
• Nonexistent valid UUID rejection
• Conflict validation between --last and session ID
• Prompt forwarding when using --last <PROMPT>

All existing tests pass, and the behavior now matches the interactive codex resume command which already had proper error handling.

[dkhokhlov]

@codex review

[chatgpt-codex-connector]

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

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