fix: add error throw for invalid extends entries

[aim4ik11]

What/Why/How?

Currently if we have invalid extends they come to bundle-extends as undefined and later while tryoing to resolve as refs trigger

➜  redocly-cli git:(main) ✗ npx redocly lint openapi.yaml --config test/redocly.yaml
Something went wrong when processing :

  - Cannot read properties of undefined (reading 'startsWith')

This causes not user-friendly error when loadConfig is used in portal build

Reference

Issue

Testing

New error log

Something went wrong when processing :

  - Found 2 unresolvable "extends" entries at position(s) [1, 3]. Each "extends" entry must be a resolvable preset name, file path, or URL.

Check yourself

• [] Code changed? - Tested with Redoc/Realm/Reunite (internal)
• [] All new/updated code is covered by tests
• [] New package installed? - Tested in different environments (browser/node)
• [] Documentation update considered

Security

• The security impact of the change has been considered
• Code follows company security practices and guidelines

[changeset-bot]

⚠️ No Changeset found

Latest commit: 22b3140

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[github-actions]

Coverage Report

Status	Category	Percentage	Covered / Total
🔵	Lines	79.16% (🎯 79%)	6383 / 8063
🔵	Statements	78.6% (🎯 78%)	6595 / 8390
🔵	Functions	82.5% (🎯 82%)	1311 / 1589
🔵	Branches	70.97% (🎯 70%)	4342 / 6118

File Coverage

File	Stmts	Branches	Functions	Lines	Uncovered Lines
Changed Files
packages/core/src/config/bundle-extends.ts	77.77%	68.75%	83.33%	82.35%	28-32, 45

Generated in workflow #8823 for commit 22b3140 by the Vitest Coverage Report Action

[github-actions]

CLI Version	Mean Time ± Std Dev (s)	Relative Performance (Lower is Faster)
cli-latest	3.773s ± 0.054s	▓ 1.01x
cli-next	3.743s ± 0.036s	▓ 1.00x (Fastest)