fix: log unsafe URL fallback at warn level

[vedansh0410]

Description

No new dependencies are required.

When UrlUtil.isSafeUrl(String) is called without a VaadinService available on the current thread, the URL safety check falls back to the default safe URL schemes. This fallback was previously logged at DEBUG level.

This change raises the log level from DEBUG to WARN so that application developers have a better chance of noticing when the configured URL safety schemes cannot be applied.

A test has been added to verify that the fallback behavior still uses the default safe URL schemes when no VaadinService is available.

Fixes #25086

Type of change

• Bugfix
• Feature

Checklist

• I have read the contribution guide: https://vaadin.com/docs/latest/guide/contributing/overview/
• I have added a description following the guideline.
• The issue is created in the corresponding repository and I have referenced it.
• I have added tests to ensure my change is effective and works as intended.
• New and existing tests are passing locally with my change.
• I have performed self-review and corrected misspellings.

Additional for Feature type of change

• Enhancement / new feature was discussed in a corresponding GitHub issue and Acceptance Criteria were created.

[cla-assistant]

All committers have signed the CLA.

[cla-assistant]

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[vedansh0410]

Hi, the TeamCity validation checks appear to be failing with user: vedansh0410 is not a flow team member. Could you please check whether this is related to the CI permissions/configuration for external contributors? The change and tests pass locally.

[Legioth]

We're considering a slightly more elaborate approach instead: #25185

[mshabarov]

Closing this PR in a favor of the mentioned one. Thanks for the contribution nevertheless!