Introduce DmfsTaskFieldBuilder

[sunkup]

Introduce DmfsTaskFieldBuilder and extract title field mapping to TitleBuilder

[Copilot]

Pull request overview

Introduces a small “field builder” abstraction for dmfs task row mapping and extracts the task title mapping into a dedicated TitleBuilder, aligning behavior with existing calendar field builders.

Changes:

• Add DmfsTaskFieldBuilder interface for mapping individual Task fields into an Android Entity.
• Extract Tasks.TITLE mapping into mapping/tasks/builder/TitleBuilder (blank/whitespace-only titles become null via trimToNull()).
• Add Robolectric unit tests covering the title mapping behavior.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

File	Description
lib/src/main/kotlin/at/bitfire/synctools/mapping/tasks/DmfsTaskBuilder.kt	Switches title mapping to the new field-builder mechanism and applies built values via withValues().
lib/src/main/kotlin/at/bitfire/synctools/mapping/tasks/builder/DmfsTaskFieldBuilder.kt	Defines the task field builder interface and its contract (including explicit null handling).
lib/src/main/kotlin/at/bitfire/synctools/mapping/tasks/builder/TitleBuilder.kt	Implements Tasks.TITLE mapping using trimToNull() to normalize blank titles to null.
lib/src/test/kotlin/at/bitfire/synctools/mapping/tasks/builder/TitleBuilderTest.kt	Adds tests for missing/blank/text SUMMARY → TITLE mapping behavior.