feat(framework): Enable user-specified include/exclude for FAB build

[chongshenng]

This PR:

• Removes .gitignore based include/exclude rules for flwr build. FAB contents are computed from user include/exclude rules and then constrained by the built-in FAB include/exclude safeguards.
• Raise errors for pattern issues:
  • fab-include or fab-exclude set to an empty list
  • A pattern that is syntactically invalid
  • A pattern that matches no files
  • fab-include patterns whose matched files are all removed by built-in constraints
• If fab-include and fab-exclude patterns overlap, fab-exclude patterns prevail and the matching files are filtered out.
• Fixes a KeyError for Windows-style path separators in build_fab_from_files: all keys are now normalized to forward slashes up-front, so callers passing src\client.py-style keys are handled correctly.
• Adds tests covering the new behavior and exclusions.

Merge after:

• feat(framework): Validate FAB include/exclude pattern lists in app config #6803

[Copilot]

Pull request overview

This PR updates FAB bundling (flwr build) so that file selection is driven by user-provided [tool.flwr.app].fab-include / fab-exclude patterns (when configured), and then constrained by the built-in FAB include/exclude safeguards. It also standardizes how warnings/notes are emitted for pattern issues and adds tests for the updated behavior.

Changes:

• Remove .gitignore-based filtering from FAB build; rely on user include/exclude patterns plus built-in constraints.
• Add unified warning/note emission for empty lists, unmatched patterns, include/exclude overlap, and built-in constraint removals.
• Add tests validating filtering outcomes and warning output.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
framework/py/flwr/cli/build.py	Reworks FAB path filtering to apply user patterns and built-in constraints; adds consistent warning/note emission.
framework/py/flwr/cli/build_test.py	Adds helper and tests for new filtering behavior and warning messages.

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