lint-mdx.js: multi-line opening tags (Card, CardGroup, etc.) silently skipped by attribute check

## Description
The opening-tag check in scripts/lint-mdx.js (checkMintlifyComponents) matches each line against a regex that expects the tag and its attributes on the same line. When a tag's name is on its own line, with attributes like title= spread across the following lines, the regex never matches - so the tag is silently skipped entirely. No required-attribute check runs, no CardGroup cols check runs, and it is not tracked for parent/child nesting rules.

## Evidence
Found 5 multi-line opening tags across 2 files that were never being checked at all:

- docs/base-account/quickstart/ai-tools-available-for-devs.mdx - 3 multi-line Card tags (lines 10, 47, 67)
- docs/apps/resources/templates.mdx - 2 multi-line Card tags (lines 22, 28)

All 5 happen to already have a valid title= attribute, so this gap doesn't currently produce any wrong output in the lint report - but it means the linter would silently miss a genuinely missing title on any of these tags, or on any future multi-line tag written in this style.

Example (ai-tools-available-for-devs.mdx):
A Card tag opens on its own line, with title= and other attributes set on the following lines, closing several lines later. Because the check only reads the line containing is found, then runs the existing regex against the accumulated text. Verified against all 5 real instances (no new findings, since they're all valid) and a synthetic multi-line Card missing title=, which is now correctly flagged.

Start in scripts/lint-mdx.js at checkMintlifyComponents and inspect how each line is matched against the opening-tag regex. Run the linter against the cited MDX files, then verify that multi-line Card tags are checked, including a synthetic Card missing title=, while existing valid tags produce no new findings.

Written by the indexing model from the issue text.