feat: add support for slotted header and footer content

[ugur-vaadin]

Description

Implements dynamic slot detection for header-content and footer slots, enabling users to pass content directly via slots instead of only through renderers.

Key changes:

• Add SlotObserver to monitor header-content and footer slots
• Dynamically set has-header/has-footer attributes based on both renderers AND slotted content presence
• Smart content detection that ignores empty elements and renderer-created containers
• Support for mixed usage (renderers + slots simultaneously)

Tests:

• Add 17 new unit tests for slotted content scenarios
• Add 8 visual regression tests
• All existing tests continue to pass (196 total)

Fixes #10339

Type of change

• Bugfix
• Feature

Checklist

• I have read the contribution guide: https://vaadin.com/docs/latest/contributing/pr
• 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.
• Enhancement / new feature was discussed in a corresponding GitHub issue and Acceptance Criteria were created.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud