fix(core): preserve request error messages without stacks

[lprnmns]

What does this PR do?

Request.pushErrorMessage() can append null or undefined when an Error has no stack. errorMessages is a persisted string[], so the invalid entry can make request reconstruction/persistence fail.

When a stack is unavailable, preserve the existing message as the fallback. This keeps stack-rich errors unchanged and prevents malformed error-message arrays.

Fixes #4087

Testing

• corepack pnpm exec vitest run test/core/request.test.ts --coverage.enabled=false
• corepack pnpm exec oxfmt --check packages/core/src/request.ts test/core/request.test.ts
• corepack pnpm exec oxlint packages/core/src/request.ts test/core/request.test.ts --tsconfig=tsconfig.json --type-aware

The full test TypeScript check was also attempted; it is currently blocked in a pristine checkout by pre-existing missing generated @crawlee/*/dist declarations and unrelated strictness errors.