chore: bump version to 0.1.15

[BugenZhao]

Purpose

Bump the Rust crate and Python package versions from 0.1.14 to 0.1.15 in preparation for the next PyPI release. After this PR lands, pushing the v0.1.15 tag from the merged main commit will trigger the existing Buildkite release pipeline.

Test Plan

• Run the locked Rust dependency check.
• Build the Python sdist and ABI3 wheel from the source tree.
• Install the wheel into a clean Python 3.12 environment and import both the Python package and Rust extension.

Test Result

• cargo check --locked passed.
• uvx --python 3.12 --from build pyproject-build produced vllm_router-0.1.15.tar.gz and vllm_router-0.1.15-cp38-abi3-macosx_11_0_arm64.whl.
• Clean wheel smoke test passed; vllm_router.__version__ reported 0.1.15 and vllm_router_rs imported successfully.