fix(isMimeType): allow underscores in parameterized MIME types

[Jerry-val]

isMimeType('text/plain; charset=ANSI_X3.4-1968') and isMimeType('multipart/form-data; boundary=----Part_123') currently return false. Underscores are valid in these values: the charset is an IANA alias, and RFC 2046 explicitly includes _ in multipart boundary characters.

Add _ to the text/multipart subtype and quoted/unquoted parameter value character classes. This also makes parameterized subtype handling consistent with the simple MIME pattern, which already accepts underscores.

This extends the cases covered by merged PR #2120: its actual change only updated the simple MIME pattern. I checked related open/closed MIME, charset and boundary PRs and found no matching implementation.

Validation:

• All nine new valid inputs returned false before the fix; both added regression tests failed.
• npm test: 325 passing, including Node/ES/browser builds and ESLint.
• Node, browser and minified-browser distributions: 12 additional assertions passed.
• git diff --check: passed.

References: RFC 2046 section 5.1.1, RFC 6838 section 4.2, IANA character sets.

Checklist

• PR contains only related changes.
• Tests written.
• References provided.
• README update is not applicable: the documented MIME-format contract is unchanged.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (a79ff98) to head (85029bf).

Additional details and impacted files

@@            Coverage Diff            @@
##            master     #2881   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files          114       114           
  Lines         2599      2599           
  Branches       658       658           
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  Hits          2599      2599           

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[nrps9909]

Verified exact head 85029bfc67e4a00c181c72606e168bc7edf7f399 against base a79ff980ab14257e795332989e497bdff3218e87.

Both added tests fail on the unchanged base; head passes full npm test (build, ESLint and 325 tests) on Node 24.15.0. All 13 reported upstream checks pass.

Independent checks cover underscore positions in parameter values, quoted/unquoted values at 1/2/69/70/71 characters, subtype positions around the existing 100-character implementation limit, IANA charset aliases, both multipart parameter orders, malformed quotes and unchanged supported/unsupported formats. Each of source, Node, browser and minified-browser forms runs 1,900 compatibility cases: base has 1,260 false rejects per form, head has zero mismatches in that scoped corpus.

The intended extension is supported by RFC 2046 §5.1.1, RFC 6838 §4.2, and the IANA charset aliases. The corpus preserves existing implementation limits; it is not an exhaustive RFC oracle.

I separately confirmed four unchanged limitations on both versions: an empty boundary and a quoted boundary ending in a space are accepted, while a quoted a:b boundary and a 101-character alphabetic subtype are rejected. These existing discrepancies are outside this underscore-only patch and are not represented as fixed or newly introduced. No objection to the scoped change.

AI-assisted review with independently executed validation; both worktrees are clean.