* fix: handle non-ISO-8859-1 characters in fetch headers and Content-Disposition
Browser Headers API rejects characters above U+00FF. The x-opencode-directory header carries raw filesystem paths, which breaks when paths contain Chinese/CJK characters. Also fixes Content-Disposition for non-ASCII filenames per RFC 5987.
* refactor: export header sanitization helpers, deduplicate, add tests
Export isLatin1Safe and sanitizeHeadersForBrowser from runtime-fetch.ts so VS Code webview can import them instead of duplicating the logic. Add tests: isLatin1Safe boundary checks, sanitizeHeadersForBrowser encoding/deduplication, runtimeFetch round-trip encode/decode, and Content-Disposition RFC 5987 output for both ASCII and non-ASCII filenames.
* fix: mark encoded directory headers
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Co-authored-by: Bohdan Triapitsyn <artmore@protonmail.com>
