chore: prepare release v0.8.0

[franciscojavierarceo]

Summary

Prepare v0.8.0 by updating the Cargo workspace, core dependency, and all workspace lockfile versions. Move unreleased changes into a dated changelog section, retain Rust API migration notes, and add the OpenTelemetry foundation, Messages usage accounting, Python packaging, and container fixes.

Published install examples will be updated separately after crates.io, PyPI, and container artifacts are verified.

Test Plan

• pre-commit run --all-files passed locally.
• cargo update --workspace --offline changed only the five workspace package versions; the core dependency matches 0.8.0.
• The preparation workflow passed workspace checking, formatting, Clippy, and tests with the same version bump, then failed because repository rules prevent its bot from creating a branch: https://github.com/vllm-project/agentic-api/actions/runs/35460824412.
• All 19 PR checks passed, including Rust, Python 3.10–3.13, the three-platform release wheel matrix, container smoke, both harnesses, provider conformance, PostgreSQL, and website validation. Publication follows a crates dry run and validation of the merged release commit.