Wire exclude-newer policy into libmambapy Database

[jezdez]

Description

Updates the libmamba solver backend for conda/conda#15761's conda-owned ExcludeNewerPolicy.

This PR now consumes the resolved policy from conda instead of reparsing raw context.exclude_newer in the solver plugin. That keeps duration/date parsing, date-only semantics, zero-duration behavior, and command-time now resolution centralized in conda.

Changes

• Declare supports_exclude_newer_global = True on LibMambaSolver.
• Leave package overrides unsupported, so conda continues to fail closed for active exclude_newer_package policies.
• Pass self.exclude_newer_policy into LibMambaIndexHelper.
• Convert the resolved global cutoff to libmambapy's exclude_newer_timestamp only when a global cutoff is active.
• Temporarily test against jezdez/conda@feature/repodata-filter-cooldown until Add first-party --exclude-newer policy conda#15761 lands.
• Add focused tests for resolved cutoff conversion and solver capability flags.

Limitations

This remains draft work because native libmamba support is still needed. It depends on mamba-org/mamba#4228 adding exclude_newer_timestamp to Database::Settings / libmambapy.

This PR supports the global cutoff only. Per-package overrides require upstream libmamba/libmambapy support for package-specific cutoffs before this solver can set supports_exclude_newer_package = True.

Part of conda/conda#15759.

Validation

• uv run --no-project --with ruff ruff check conda_libmamba_solver/index.py conda_libmamba_solver/solver.py tests/test_index.py tests/test_solver.py
• uv run --no-project --with ruff ruff format --check conda_libmamba_solver/index.py conda_libmamba_solver/solver.py tests/test_index.py tests/test_solver.py
• PYTHONPATH=/Users/jezdez/Code/git/conda:. /Users/jezdez/miniforge3/envs/cls-dev/bin/python -m pytest -o addopts='' tests/test_index.py::test_exclude_newer_timestamp_unset tests/test_index.py::test_exclude_newer_timestamp_uses_resolved_global_cutoff tests/test_solver.py::test_solver_declares_global_exclude_newer_support_only -q
• PYTHONPATH=/Users/jezdez/Code/git/conda:. /Users/jezdez/miniforge3/envs/cls-dev/bin/python -m compileall -q conda_libmamba_solver/index.py conda_libmamba_solver/solver.py tests/test_index.py tests/test_solver.py
• git diff --check

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