fix(core): resolve native TS7 runtime via exports and cover adapter

[tazo90]

Summary

Follow-up to #162 (native TS7 support). The Coverage gate failed on main because the new
native-typescript-adapter.ts (~1300 LOC) shipped with 3.62% branch coverage, dragging the openapi-core scope below its 73% branch threshold.

Changes

• typescript-runtime.ts: Remove the dead "unsupported-native" member of TypeScriptVersionSupport and its unreachable message branch (never returned by getTypeScriptVersionSupport).
• typescript-runtime.ts: Resolve the TS7 native runtime through the package's public exports map (typescript/unstable/sync, typescript/unstable/ast) instead of hardcoded dist/... paths, so internal layout changes in TS7 RCs no longer break resolution.
• native-typescript-adapter.test.ts (new, 27 tests): Cover evaluateNode, inferResponsesForExports, resolveTypeByName / typeToOpenApiSchema, resolveModule, and project lifecycle via a fake TS7 runtime. Lifts openapi-core branch coverage from 68.12% to 73.49% (threshold 73%).
• getting-started.md: Document the schemaType guard tightening introduced in feat: native ts7 support and multipart improvement #162 (TS checker fallback is skipped when schemaType does not include "typescript").

Verification

• pnpm exec turbo run lint typecheck --filter=@workspace/openapi-core: 0 warnings, 0 errors.
• COVERAGE_SCOPE=openapi-core vitest run --coverage: All files 82.89 / 73.49 / 92.44 / 84.33 (branches 73.49% >= 73 threshold), 840/840 tests pass.
• Existing TS7-mock tests ( ypescript-runtime, ypescript-project, ypescript-response-inference) still pass against the new exports-based resolution.

Type of change

• fix: Bug fix (non-breaking change which fixes an issue)

Checklist

• pnpm check passes
• Tested locally (pnpm test and coverage gate)
• Documentation updated