Make mr-iid optional: auto-select single MR or prompt interactively

[Copilot]

mr:diff, mr:files, mr:status, and mr:logs all require mr-iid as a positional argument, which is redundant noise when an issue has only one open MR.

Changes

• MrCommandBase::initialize() — when mr-iid is omitted on the NID path, calls ListMergeRequestsAction to fetch open MRs and applies the following logic:

State	Behaviour
Zero open MRs	RuntimeException with clear message
One open MR	Auto-selects; prints Auto-selected MR !N: title
Multiple MRs, non-interactive	Lists !iid — title to stderr, throws asking user to specify
Multiple MRs, interactive	ChoiceQuestion on !iid — title (same pattern as issue:checkout)

• MrCommandBaseTest — verifies both nid and mr-iid arguments remain OPTIONAL.

Example

# Before: always required
drupalorg mr:diff 3383637 7

# After: omit mr-iid when there's only one open MR
drupalorg mr:diff 3383637
# → Auto-selected MR !7: Fix the regression in node access

Original prompt

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This section details on the original issue you should resolve

<issue_title>Make mr-iid optional: auto-select single MR or prompt interactively</issue_title>
<issue_description>## Context

PR #293 added the mr:diff, mr:files, mr:status, and mr:logs commands. All of them require mr-iid as a positional argument. In practice, most issues have only one open MR, making the argument redundant noise.

Proposed behaviour

• Zero MRs — exit with a clear error message.
• One open MR — use it automatically (no argument needed).
• Multiple open MRs — in interactive mode, present a choice prompt (same pattern as issue:checkout branch selection); in non-interactive mode, list the available IIDs and exit with an error asking the user to specify one.

Implementation notes

• MrCommandBase::initialize() already resolves the issue NID and has access to $this->client and a GitLabClient. It can call ListMergeRequestsAction to fetch open MRs when mr-iid is not supplied.
• mr-iid must stay InputArgument::OPTIONAL (Symfony disallows a required argument after an optional nid), so the runtime validation in initialize() is the right place to add the auto-selection logic.
• The interactive prompt should use ChoiceQuestion on $mr->iid — $mr->title, consistent with the issue:checkout pattern.

Follow-up to

PR #293</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Make mr-iid optional: auto-select single MR or prompt interactively #295

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