Replace is_unknown with is_und

[sffc]

Maybe fixes #6505

@gemini-code-assist There is some disagreement about whether this function should be called "is_unknown" or "is_und". Would you like to weigh in?

[Copilot]

Pull Request Overview

This PR renames the function and its references from is_unknown to is_und throughout the codebase to address naming consistency and resolve #6505.

• Renames method names and related calls in multiple modules such as provider/core, provider/blob, and components/locale.
• Updates inline documentation and test assertions accordingly.

Reviewed Changes

Copilot reviewed 446 out of 451 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
provider/core/src/request.rs	Updated function name and corresponding call.
provider/blob/src/blob_schema.rs	Replaced is_unknown with is_und in condition checks.
provider/baked/src/export.rs	Changed all locale.is_unknown() calls accordingly.
provider/adapters/src/fallback/mod.rs	Updated function call in fallback loop.
ffi/capi/src/locale_core.rs	Updated documentation link to use is_und.
ffi/capi/src/fallbacker.rs	Renamed check in next() method for locale.
components/locale_core/tests/langid.rs	Updated test assertion to check is_und.
components/locale_core/src/subtags/language.rs	Renamed method from is_unknown to is_und.
components/locale_core/src/preferences/locale.rs	Updated conditional check for language preference.
components/locale_core/src/langid.rs	Replaced is_unknown with is_und in identifier check.
components/locale_core/src/data.rs	Updated API to use is_und in method and test examples.
components/locale/src/fallback/mod.rs	Updated doc comments for fallback loop reference.
components/locale/src/fallback/algorithms.rs	Replaced is_unknown with is_und in debug_assert calls.
components/locale/src/expander.rs	Updated all checks and conditions to use is_und.
components/locale/src/canonicalizer.rs	Replaced is_unknown checks in canonicalization functions.

Files not reviewed (5)

• provider/data/calendar/data/calendar_chinese_v1.rs.data: Language not supported
• provider/data/calendar/data/calendar_dangi_v1.rs.data: Language not supported
• provider/data/calendar/data/calendar_hijri_simulated_mecca_v1.rs.data: Language not supported
• provider/data/calendar/data/calendar_japanese_extended_v1.rs.data: Language not supported
• provider/data/calendar/data/calendar_japanese_modern_v1.rs.data: Language not supported

[gemini-code-assist]

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