fix(core): detect kmscon terminal as supporting true color

[claygeo]

Summary

• Adds COLORTERM=kmscon recognition to supports256Colors() and supportsTrueColor() in compatibility.ts
• kmscon (Fedora's new virtual console) sets TERM=linux and COLORTERM=kmscon, which the existing checks didn't recognize, causing a false "256-color support not detected" warning
• Adds unit tests for kmscon detection in both color functions and an integration test in getCompatibilityWarnings

Fixes #25210

Test plan

• npx vitest run packages/core/src/utils/compatibility.test.ts — all 39 tests pass
• npx tsc -p packages/core/tsconfig.json --noEmit — clean
• Manual verification on a kmscon terminal (Fedora virtual console) confirms no false warning

[gemini-code-assist]

Summary of Changes

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This pull request enhances the terminal color compatibility detection logic to correctly identify kmscon terminals as supporting true color. This change prevents erroneous warnings about a lack of 256-color support on systems using kmscon, improving the user experience for Fedora virtual console users by ensuring accurate color capability reporting.

Highlights

• KMSCON Terminal Detection: Updated color compatibility detection to recognize COLORTERM=kmscon for both 256-color and true-color support.
• Warning Resolution: Resolved an issue where kmscon terminals, which set TERM=linux and COLORTERM=kmscon, incorrectly triggered a "256-color support not detected" warning.
• Unit Tests: Introduced new unit tests to verify kmscon detection in supports256Colors() and supportsTrueColor().
• Integration Test: Added an integration test to getCompatibilityWarnings() to ensure no false color warnings are issued for kmscon terminals.

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[gemini-code-assist]

Code Review

This pull request adds support for the kmscon terminal emulator, which supports true color despite setting its TERM variable to linux. The changes include updates to the color compatibility utility functions and the addition of comprehensive test cases to verify correct behavior. A review comment suggests refactoring the 256-color support check to internally call the true-color support function, which would reduce code duplication and ensure that any terminal capable of true color is automatically recognized as supporting 256 colors.