fix(vite): merge user resolve.conditions instead of overwriting them

[wook10]

Summary

@tailwindcss/vite's CSS resolver hardcodes conditions: ['style', 'development|production'] in two places in packages/@tailwindcss-vite/src/index.ts (the pre-Environment-API branch and the newer Environment API branch). This overwrites any resolve.conditions the user configured in their Vite config instead of merging with them, so a package that relies on a custom export condition for its CSS entry point silently falls back to the default/style export.

This PR appends the user's configured resolve.conditions (if any) to the built-in conditions at both call sites instead of replacing them.

Test plan

• Added a regression test in integrations/vite/resolvers.test.ts using a local workspace package with a conditional CSS export, asserting that a custom resolve.conditions entry is honored by the CSS resolver during a production build.
• CI (vite integration job) — I wasn't able to build @tailwindcss/oxide locally (no Rust toolchain in my current environment) to run the full integration suite, so I'd appreciate CI confirming this on all platforms.

Fixes #20426

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

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Configuration used: Repository UI

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Run ID: af38a03d-e4e5-490b-b139-bc12a67a1fac

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Reviewing files that changed from the base of the PR and between a121d65 and dab4ebe.

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• integrations/vite/resolvers.test.ts

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Walkthrough

The Vite plugin now appends user-defined resolve.conditions to its built-in CSS resolver conditions in legacy and environment-based paths. A production-build integration test runs with Vite ^8 and ^5.3.5 and verifies that the custom stylesheet is selected.

Merge Risk: ⚪ Minimal · up to dab4e

The PR preserves Vite's built-in CSS resolution conditions while honoring user-configured conditions, with a regression test covering the affected behavior; no actionable merge-blocking risk remains after normal checks and review.

🚥 Pre-merge checks | ✅ 4

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly summarizes the main change: merging user Vite resolve conditions instead of overwriting them.
Description check	✅ Passed	The description accurately explains the resolver bug, the fix, the regression test, and the linked issue.
Linked Issues check	✅ Passed	The implementation preserves user resolve conditions in both CSS resolver branches and adds regression coverage for both supported paths [#20426].
Out of Scope Changes check	✅ Passed	The changes are limited to merging Vite resolver conditions and testing custom conditional CSS exports.

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[greptile-apps]

Confidence Score: 5/5

The PR appears safe to merge.

The previously reported legacy resolver coverage gap is addressed by an independent Vite 5.3.5 integration case, and no blocking failure remains.

_{Reviews (2): Last reviewed commit: "test(vite): cover legacy (pre-Environmen..." | Re-trigger Greptile}

[wook10]

Good catch — the regression test only covered the Environment API path. Pushed a follow-up commit parameterizing it over Vite 8 and Vite 5.3.5 so both resolver branches are exercised.