Adds license warnings to the BFF

[Erwinvandervalk]

What issue does this PR address?

This PR warns on license enforcement but doesn't actually implement any enforcement yet. This will happen on final release of V4.

• Warns on no license
• Warns on invalid license
• Warns on expired license
• Warns if license doesn't contain BFF Feature
• Displays message on valid log message
• Messages when frontend is added within license boundaries
• Messages when frontend is added which exceeds number of frontends

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(see our license, section 7)

[Copilot]

Pull Request Overview

This PR implements license warnings for the BFF (Backend for Frontend) framework by replacing the existing license validation system with a new warning-based approach. The implementation warns users about license issues without enforcing them yet, in preparation for V4 release.

Key changes include:

• Replaces the existing license validation logic with a warning-only system
• Adds comprehensive licensing warnings for various scenarios (no license, expired license, invalid license, frontend limits)
• Integrates license validation into frontend collection management

Reviewed Changes

Copilot reviewed 17 out of 17 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
bff/src/Bff/Licensing/LicenseValidator.cs	Complete rewrite of license validation logic to use warning-based approach instead of enforcement
bff/src/Bff/Licensing/LicensingLogMessages.cs	New file containing structured log messages for various license scenarios
bff/src/Bff/Licensing/License.cs	Removed legacy license class implementation
bff/src/Bff/DynamicFrontends/Internal/FrontendCollection.cs	Integrates license validator to log warnings when frontends are added
bff/test/Bff.Tests/LicensingTests.cs	New comprehensive test suite covering all license warning scenarios

Comments suppressed due to low confidence (2)

bff/test/Bff.Tests/LicensingTests.cs:98

• [nitpick] The method name should be consistent with other test method names. Consider using 'warns' instead of 'warning' to match the pattern used in other test methods.

    public async Task When_exceeding_number_of_licensed_frontends_then_warning()

[maartenba]

Couple of suggestions (also depending on https://github.com/DuendeSoftware/fenix/pull/508)