fix(cli): preserve double dash before entrypoint

[Tiancheng-Xu]

Summary

• preserve -- as an end-of-options separator before the first positional entrypoint
• continue routing arguments after the entrypoint to the command trailing arguments
• add focused parser regressions for run, eval, and root-script forms

Validation

• cargo test -p deno_cli_parser --lib --offline --features deno_core/v8: 379 passed
• debug deno binary built successfully
• focused CLI smoke matrix for eval --, run --, leading-hyphen scripts, and trailing arguments passed
• cargo fmt --all -- --check passed
• git diff --check passed

./x check was stopped after it expanded into a full-repository compile beyond this two-file parser change.

Fixes #36792

AI assistance disclosure

This PR was prepared with assistance from ChatGPT/Codex.

[Copilot]

🟡 Changes recommended

The new positional_only path diverges from existing trailing/---stripping and unexpected-positional error behavior, which can mis-handle argv in real CLI invocations.

Once you've addressed the issues Copilot identified, you can request another Copilot review.

Pull request overview

This PR updates the libs/cli_parser argument parser to treat -- as an end-of-options separator before the first positional “entrypoint” (ex: deno run -- -echo.ts), restoring behavior needed for leading-hyphen scripts/code and keeping post-entrypoint arguments routed to trailing args.

Changes:

• Add a positional_only parsing mode to consume initial positionals after -- without interpreting leading - as flags.
• Preserve existing trailing-arg behavior after the entrypoint (including keep_double_dash semantics).
• Add regression tests covering run -- <script> and eval -- <code>.

File summaries

File	Description
libs/cli_parser/src/parse.rs	Implements positional_only parsing after -- prior to the first positional.
libs/cli_parser/src/tests.rs	Adds parser regressions for run and eval using -- before entrypoint/code.

Review details

Suppressed comments (1)

libs/cli_parser/src/parse.rs:192

• When positional_only is active and there are more argv tokens than there are positional defs, the parser currently pushes the extras into result.trailing unconditionally. That differs from the normal parsing path (which errors with UnexpectedPositional when trailing_var_arg is false) and can cause commands like deno info -- -file.ts extra to be accepted unexpectedly.

      result.trailing.push(arg.clone());
      i += 1;
      continue;

• Files reviewed: 2/2 changed files
• Comments generated: 2
• Review effort level: Lite

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[Tiancheng-Xu]

Addressed the Copilot review in commit 6ab567a9: positional-only parsing now returns UnexpectedPositional when trailing_var_arg is disabled, matching the normal parser path. Added a regression covering deno upgrade -- v1 v2. git diff --check passes; cargo is unavailable in this environment, so I could not rerun the Rust test target locally. The PR already includes the required AI-assistance disclosure.