Implemented PassSuppressFormatter, a custom Behave formatter that buffers
per-scenario output and only flushes it to stdout when the scenario failed
or errored. Passing scenarios produce no output, keeping an all-passing
suite at ~5-10 lines (the _print_overall_summary block only).

Key design decisions:
- PassSuppressFormatter inherits from behave's Formatter base class so it
  can be registered via behave.formatter._registry.register_as(), which
  enforces issubclass(cls, Formatter).
- _SUPPRESS_STATUSES = {'passed', 'skipped'} determines which terminal
  statuses are silently discarded; all others (failed, undefined, etc.)
  trigger a buffer flush so no failure is hidden.
- _make_runner() registers the formatter and sets it as the default format
  whenever config.format is None (no explicit -f/--format flag). Coverage
  mode (BEHAVE_PARALLEL_COVERAGE=1) bypasses the formatter and falls back
  to config.default_format so slipcover can instrument a single process.
- The formatter is embedded directly in run_behave_parallel.py (not a
  separate file) so the noxfile's _install_behave_parallel() packaging
  step, which copies only run_behave_parallel.py, picks it up with zero
  noxfile changes.
- Three new BDD scenarios in behave_parallel_log_filtering.feature cover:
  (1) passing scenario -> no output, (2) failing scenario -> full output,
  (3) mixed run -> only failing scenario visible. All 20 scenarios pass.

ISSUES CLOSED: #10987

@@ -34,0 +34,4 @@
+from behave.formatter.base import (
+    Formatter as _BehaveFormatter,  # type: ignore[import-untyped]
+)
+

@@ -34,1 +34,4 @@
+        PassSuppressFormatter,
+    )
+
 DEFAULT_FEATURE_ROOT = "features/"

@@ -360,0 +404,4 @@
+    to the formatter's real output stream after simulating scenario events.
+    """
+    from behave.configuration import Configuration  # type: ignore[import-untyped]
+    from behave.formatter.base import StreamOpener  # type: ignore[import-untyped]

@@ -360,0 +404,4 @@
+    to the formatter's real output stream after simulating scenario events.
+    """
+    from behave.configuration import Configuration  # type: ignore[import-untyped]
+    from behave.formatter.base import StreamOpener  # type: ignore[import-untyped]