fix(logging): drop dead _HFSymlinksInfoFilter and redundant TasksManager demote filter

[timenick]

Closes #909.

_warnings.py had two "demote WARNING→INFO" logging filters that relabel records but don't actually suppress them. The CLI uses the root logger level as the sole emit-time gate (handler is NOTSET), so a filter that rewrites levelno after the gate has passed cannot hide a record — it only changes the printed label.

_HFSymlinksInfoFilter was dead code

• fix(_warnings): downgrade huggingface_hub symlinks UserWarning to INFO #647 added it on the py.warnings logger to demote the huggingface_hub symlinks UserWarning to INFO.
• fix: suppress ORT native stderr, fix HANDLE bug, clean up warnings #709 later added warnings.filterwarnings("ignore", message=r".*huggingface_hub.*cache-system.*symlinks.*"), which drops the warning at the Python warnings layer before captureWarnings could route it to py.warnings. The demote filter never executed.
• Net: the warning is fully suppressed in every verbosity mode; the filter did nothing.

_TasksManagerFilter was redundant

• It demoted optimum's "TasksManager returned …" WARNING on optimum.exporters.tasks.
• fix(cli): quiet third-party optimum logger noise in normal output #904 added a verbosity-conditional ERROR floor on the parent optimum logger in configure_logging. Because optimum.exporters.tasks inherits its effective level from optimum, the notice is already gated at the source: hidden by default, shown at -v/-vv. The demote filter only relabeled the record at -v; it provided no suppression.

Changes

• Remove both filter classes and their addFilter registrations in src/winml/modelkit/_warnings.py.
• Keep the filterwarnings("ignore") suppressor; refresh its comment and add a NOTE that optimum's informational WARNINGs are gated by the ERROR floor in utils/logging.py (so no demote filter is needed and one must not be re-added).
• Replace the obsolete _HFSymlinksInfoFilter test in tests/unit/test_warnings_configure.py with a regression test that the HF symlinks UserWarning is suppressed (plus a specificity check that unrelated symlinks warnings pass through).
• Add test_optimum_child_logger_gated_by_parent_floor to tests/unit/utils/test_logging.py proving optimum.exporters.tasks is isEnabledFor(WARNING) False at default and True at -v — locking in floor-based gating.

Behavior

• HF symlinks warning: unchanged (fully suppressed before and after).
• TasksManager notice: hidden by default (unchanged); at -v it now prints at its native WARNING level instead of a relabeled INFO line — more truthful.

[DingmaomaoBJTU]

Both removed filters are verified dead/redundant after tracing the execution path.

_HFSymlinksInfoFilter: The filterwarnings(ignore) added in #709 suppresses the warning at the Python warnings layer, before captureWarnings(True) can route it to the py.warnings logger. The demote filter never had a record to process. Dead code confirmed.

_TasksManagerFilter: optimum.exporters.tasks inherits its effective level from the optimum parent. configure_logging (#904) pins optimum at ERROR at default verbosity, so WARNING records are rejected by level check before reaching any filter. At -v the floor drops, but the filter only relabeled records anyway. Redundant confirmed.

Remaining references: Only a clarifying comment in test_logging.py -- no live code references to either symbol.

Tests: Old tests covered the dead demote-filter behavior; new tests correctly exercise the actual suppression mechanism. All 7 tests pass.

Minor observation: _restore_logging_filters does not restore warnings module state, but both new tests use catch_warnings(record=True) + resetwarnings() which fully own the warnings stack inside their context.