Fix positional skipping for grouped conflicts with allow_missing_positional

[jeremy99981]

This PR fixes #1794.

Problem:
When allow_missing_positional(true) is used with an ArgGroup, values can still be assigned to a positional that conflicts with an already-present option from the same group. In multi-positional cases, this causes an avoidable ArgumentConflict instead of advancing to the next valid positional.

Cause:
Positional skipping logic only considered syntax cues (like whether the next token looked like an option/subcommand), not whether the current positional was already in conflict with arguments that had been parsed.

Solution:

• Update positional-counter correction in the parser to skip positionals that conflict with already-present args when allow_missing_positional is enabled.
• Include both explicit arg conflicts and same non-multiple ArgGroup membership conflicts.
• Re-enable and update issue_1794 parse-behavior test and add a new regression test for multiple conflicting positionals (pos1, pos2, option1) ensuring the value lands on pos3.

Testing:

• cargo test --test builder issue_1794 -- --nocapture
• cargo test --test builder groups::
• cargo test --test builder allow_missing_positional
• cargo fmt --all --check
• cargo test --test builder

Notes:
If this issue is still eligible for the IssueHunt bounty, I’m happy to follow the project’s normal bounty redemption process after review and merge.

[epage]

Moving to a draft as #1794 is not marked as being ready for implementation.

[jeremy99981]

Thanks for the heads-up, understood. I’ll keep this as draft until #1794 is marked ready for implementation.

[pksunkara]

@jeremy99981 Design needs to be agreed upon first in the issue. Can you please add a detailed comment about your design on that issue?