lint-mdx.js: false-positive missing-alt warning for multi-line <img> tags

## Description
The alt-attribute check in scripts/lint-mdx.js (inside checkMintlifyComponents) only inspects the single line containing tag's attributes are spread across multiple lines (a common JSX style used throughout docs/), an alt= attribute on a later line is never seen, and the linter reports a false-positive "should have alt attribute" warning.

## Evidence
Running node scripts/lint-mdx.js all on current master reports 5 such warnings, all false positives - each file already has a valid alt:

- docs/base-account/improve-ux/sponsor-gas/paymasters.mdx:33 and :57
- docs/base-account/reference/ui-elements/brand-guidelines.mdx:128 and :156
- docs/snippets/BasePayButton.mdx:21

Example from paymasters.mdx - the img tag spans multiple lines with alt="Paymaster CDP" set on its own line, not on the same line as the opening until the tag closes, matching the multi-line lookback pattern already used by the adjacent Frame-wrapping check in the same function. Verified against synthetic missing-alt and multi-line-with-alt cases to confirm real violations are still caught.