fix: preserve proper nouns in diagnostic reasons

[eoinest]

Summary

• Preserve authored casing when diagnostic reasons are combined, so proper names such as Next.js are not lowercased.
• Normalize existing sentence-fragment reasons and cover the actual gt-next warning while keeping prior diagnostic output unchanged.

Testing

• pnpm build — passed (20 packages)
• pnpm --filter generaltranslation test — passed (31 files, 499 tests)
• pnpm --filter gt-next test:js — passed (25 files, 273 tests)
• pnpm --filter @generaltranslation/react-core test — passed (11 files, 46 tests)
• pnpm --filter gt-next exec vitest run src/errors/__tests__/createErrors.test.ts — passed (1 file, 3 tests)
• pnpm exec oxlint <changed TypeScript files> — passed
• pnpm exec oxfmt --check <changed files> — passed

Notes

• Closes Diagnostic why clause lowercases proper nouns: "because next.js may select a locale" #2016.
• Changeset: patch releases for generaltranslation, @generaltranslation/react-core, and gt-next.

Greptile Summary

This PR fixes a bug in createDiagnosticMessage where the lowercaseFirstWord helper was incorrectly downcasing proper nouns (e.g. Next.js → next.js) when combining why clauses into a single sentence. The fix removes lowercaseFirstWord entirely and shifts the responsibility to each call site, which must now author why strings as sentence fragments using their intended in-sentence casing.

• diagnostics.ts: Removes lowercaseFirstWord; why and wayOut clauses are now passed directly to stripSentence. The wayOut path is effectively a no-op change since shouldCombineWayOut already required wayOut to start with a lowercase letter via /^[a-z]/.
• Call-site updates (client.ts, createErrors.ts, ssg.ts, singleton-operations.ts): Existing why clauses that started with common English words (This, The, Static, Without) are lowercased to match the new convention.
• Tests: New cases in both diagnostics.test.ts and createErrors.test.ts verify that Next.js casing is preserved end-to-end, and that the createNextI18nConfigMismatchWarning helper produces the correct full message string.

Confidence Score: 5/5

Safe to merge — the change is narrow, well-tested, and all call sites have been updated to the new convention.

The removal of lowercaseFirstWord is clean: the why path is the only place producing wrong output, all affected call sites have been updated to lowercase-start fragments, and a grep across the monorepo confirms no remaining why clause would produce an awkward combined sentence. The wayOut removal is provably inert because shouldCombineWayOut already gates on the clause starting lowercase. New tests cover both the core utility and the specific createNextI18nConfigMismatchWarning entry point that triggered the issue.

Files Needing Attention: No files require special attention.

Important Files Changed

Filename	Overview
packages/core/src/logging/diagnostics.ts	Removes lowercaseFirstWord helper and passes why/wayOut clauses directly to stripSentence; fixes proper-noun casing without behavioral regression for wayOut (already gated by a /^[a-z]/ check).
packages/next/src/errors/createErrors.ts	Adds createNextI18nConfigMismatchWarning export with proper proper-noun casing; updates two why clauses to sentence-fragment form (lowercase start).
packages/next/src/errors/ssg.ts	Two why clauses updated to lowercase sentence-fragment form, consistent with the new convention.
packages/next/src/errors/client.ts	Single why clause lowercased to match new sentence-fragment convention.
packages/react-core/src/condition-store/singleton-operations.ts	why clause lowercased to match new sentence-fragment convention.
packages/core/src/logging/tests/diagnostics.test.ts	New test case verifies proper-noun casing is preserved in combined why clauses.
packages/next/src/errors/tests/createErrors.test.ts	New test suite for createNextI18nConfigMismatchWarning validates the full message string including preserved Next.js casing.
.changeset/calm-geckos-smile.md	Patch changeset for generaltranslation, @generaltranslation/react-core, and gt-next — appropriately scoped.

Flowchart

%%{init: {'theme': 'neutral'}}%%
flowchart TD
    A["createDiagnosticMessage(input)"] --> B{why provided?}
    B -- Yes --> C["stripSentence(whatHappened) + ' because ' + stripSentence(why)"]
    B -- No --> D["whatHappened as-is"]
    C --> E{fix AND wayOut AND wayOut starts with a-z?}
    D --> E
    E -- Yes --> F["stripSentence(fix) + ', or ' + stripSentence(wayOut)"]
    E -- No --> G["fix as-is"]
    F --> H["Assemble parts via ensureSentence and join"]
    G --> H
    H --> I["Add source/severity prefix"]
    I --> J["Final diagnostic string"]

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[github-actions]

size-limit report 📦

Path	Size
generaltranslation	18.09 KB (-0.18% 🔽)
generaltranslation/runtime	14.87 KB (-0.45% 🔽)
generaltranslation/id	2.55 KB (0%)
generaltranslation/internal	7.36 KB (-0.41% 🔽)
generaltranslation/types	115 B (0%)
generaltranslation/errors	81 B (0%)
@generaltranslation/format	9.5 KB (0%)
@generaltranslation/format/types	89 B (0%)
@generaltranslation/format/internal	880 B (0%)
gt-i18n	11.88 KB (-0.23% 🔽)
gt-i18n/types	13 B (0%)
gt-i18n/internal	22.45 KB (-0.04% 🔽)
gt-i18n/internal/types	13 B (0%)
@generaltranslation/react-core/pure	25.52 KB (-0.18% 🔽)
@generaltranslation/react-core/hooks	20.59 KB (-0.09% 🔽)
@generaltranslation/react-core/components	22.67 KB (-0.58% 🔽)
@generaltranslation/react-core/components-rsc	26.15 KB (-0.05% 🔽)
gt-react (client)	31.99 KB (-0.09% 🔽)
gt-react (rsc)	28.63 KB (-0.04% 🔽)
gt-react (server)	31.62 KB (-0.16% 🔽)
gt-react/macros	8.81 KB (-0.4% 🔽)
gt-next (client)	43.83 KB (-0.16% 🔽)
gt-next (rsc)	47.77 KB (-0.3% 🔽)
gt-next (server)	44.11 KB (+0.23% 🔺)
gt-next/config	269.99 KB (-0.07% 🔽)
gt-next/server	46.4 KB (-0.01% 🔽)
gt-next/middleware	36.52 KB (-0.14% 🔽)
gt-next/link	42.88 KB (+0.28% 🔺)
gt-next/internal/_dictionary	144 B (0%)
gt-next/internal/_load-translations	144 B (0%)
gt-next/internal/_load-dictionary	144 B (0%)
gt-next/internal/_getLocale	125 B (0%)
gt-next/internal/_getRegion	122 B (0%)
gt-node	23.5 KB (-0.21% 🔽)
gt-node/types	219 B (0%)
gt-node/internal	13.45 KB (-0.16% 🔽)
gt-tanstack-start (client)	31.61 KB (-0.13% 🔽)
gt-tanstack-start (server)	32.03 KB (-0.22% 🔽)
gt-tanstack-start/server	10.24 KB (-0.28% 🔽)
gt-react-native	30.07 KB (+0.09% 🔺)
gt-react-native/plugin	4.6 KB (+0.15% 🔺)
gt-react-native/internal	746 B (0%)