…y and predicate filters (microsoft#6488)

* Python: [Breaking] Refactor FileSkillsSource for depth-based discovery and predicate filters

Refactors FileSkillsSource to make script and resource discovery more flexible.

## Changes

- **Drops** resource_directories / script_directories options (preconfigured
  directory whitelists).
- **Adds** search_depth option (>= 1, default 2): controls how deep the
  recursive scan goes within each skill directory.
- **Adds** script_filter / resource_filter predicate options that receive a
  FileSkillFilterContext (skill_name + relative_file_path), allowing
  whitelist/blacklist filtering by file path.
- **Adds** FileSkillFilterContext class exported from agent_framework.

## Notes

- The Skills API is marked @experimental -- the option removals are intentional
  breaking changes within the experimental surface.
- Security checks (path containment, symlink detection) are preserved and
  continue to use the skill root directory as the trusted boundary.
- Ports the same refactoring from .NET PR microsoft#6109 while following Python
  conventions (instance methods, Callable type hints, __slots__).

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Address PR feedback: clarify depth constants and skip nested skill directories

- Add clarifying comments distinguishing MAX_SEARCH_DEPTH (SKILL.md
  discovery) from DEFAULT_SEARCH_DEPTH (per-skill resource/script scanning).
- Stop recursing into subdirectories that contain their own SKILL.md,
  preventing child skill files from being attached to the parent skill.
- Add test verifying nested skill boundary is respected.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Remove __slots__ from FileSkillFilterContext and add type-ignore comments

- Remove __slots__ from FileSkillFilterContext per reviewer feedback —
  the optimization is negligible and inconsistent with sibling classes.
- Add type: ignore[attr-defined] / ty: ignore[unresolved-attribute]
  comments to test lines accessing private _resources/_scripts attributes,
  matching the convention established on main.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Simplify filter predicates: remove FileSkillFilterContext, use Callable[[str, str], bool]

Address reviewer feedback:
- Remove FileSkillFilterContext class — a dedicated class for two strings
  is overkill in Python. Filters now receive (skill_name, relative_file_path)
  directly as positional args.
- Update docstrings to describe behavior instead of referencing private
  instance attributes.
- Remove FileSkillFilterContext from exports and __all__.
- Update all test lambdas and remove TestFileSkillFilterContext class.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Use DEFAULT_SEARCH_DEPTH as default argument directly

Instead of accepting int | None and resolving None to the default
internally, use DEFAULT_SEARCH_DEPTH as the parameter default value
on both FileSkillsSource.__init__() and SkillsProvider.from_paths().

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

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Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>