fix(content_disposition): enforce RFC 7230 quoted-pair grammar strictly

[nao1215]

Summary

multipartkit/content_disposition.parse previously honoured every \X escape inside a quoted-string by silently dropping the backslash and keeping the second character, even when X was outside the RFC 7230 §3.2.6 quoted-pair grammar (HTAB / SP / VCHAR / obs-text). That let NUL, CR, LF, and other ASCII control bytes flow into the decoded name / filename, enabling NUL-smuggling against downstream C-string consumers. This PR aligns the parser with the RFC and rejects illegal quoted-pairs with the new Error(InvalidQuotedPair(_)).

Changes

• src/multipartkit/error.gleam: add InvalidQuotedPair(String) variant with a doc-comment that explains the grammar and the security motivation.
• src/multipartkit/internal/text.gleam: add read_token_or_quoted_strict plus its supporting QuotedFault (QuotedSyntax / QuotedPairInvalid) and a quoted-pair second-character validator that allows HTAB, SP, VCHAR (%x21-7E), and obs-text (%x80-FF) while rejecting NUL, LF, CR, the remaining %x00-1F control bytes, and DEL (%x7F). The existing lenient read_token_or_quoted / read_quoted_string is left untouched so Content-Type parsing in header.gleam keeps its current behaviour.
• src/multipartkit/content_disposition.gleam: route the parameter-value parse through the strict variant, mapping QuotedPairInvalid to Error(InvalidQuotedPair(original)) and QuotedSyntax to the existing Error(InvalidContentDisposition(original)). The module doc-comment now documents the RFC 7230 §3.2.6 contract.
• test/regression_cd_quoted_pair_test.gleam: the eight acceptance tests from issue content_disposition.parse over-eager quoted-pair unescape — drops backslash before non-VCHAR (NUL, CR, LF) silently #50 — canonical \\ and \" round-trips, the VCHAR-escape backslash-drop case, NUL / CR / LF rejection, a no-escape baseline, and a UTF-8 name regression check.
• CHANGELOG.md: one-line ### Changed entry under ## [Unreleased].

Design Decisions

Picked option A (strict RFC 7230 compliance) from the issue: rejecting bad escapes upstream means callers never see a name or filename carrying smuggled control bytes, which closes the security angle the issue called out without forcing every caller to add their own sanitiser. The strict parser lives next to the existing lenient one rather than replacing it because Content-Type parameter parsing (header.gleam) is out of scope for #50 and shares the helper — keeping both variants avoids unrelated behaviour changes there. The second-character validator enumerates the rejected ASCII code points explicitly with \u{...} literals so the grammar boundaries are obvious at the call site, and any grapheme outside that block (including all multi-byte UTF-8) is treated as obs-text and allowed, matching the RFC.

Closes #50

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Reviewing files that changed from the base of the PR and between 567591f and 2000da4.

📒 Files selected for processing (5)

• CHANGELOG.md
• src/multipartkit/content_disposition.gleam
• src/multipartkit/error.gleam
• src/multipartkit/internal/text.gleam
• test/regression_cd_quoted_pair_test.gleam

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• Create PR with unit tests
• Commit unit tests in branch fix/issue-50-quoted-pair-rfc7230

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