chore(release): prepare coordinated Ack 1.6.2

[leoafarias]

Coordinated Ack 1.6.2 release preparation

Prepare all six publishable packages for one shared v1.6.2 tag, following PUBLISHING.md. This PR does not create a tag or publish packages.

• Set ack, ack_annotations, ack_generator, ack_firebase_ai, ack_json_schema_builder, and ack_mcp_dart to 1.6.2.
• Move the JSON Schema import compatibility change from Unreleased to the core 1.6.2 notes; add alignment entries for the other five packages and update the workspace changelog.
• Set release preflight API baseline to v1.6.1 and update release guide commands and installation examples; clarify that the MCP first-publication procedure applied to v1.3.0 only.
• Keep existing compatible hosted dependency minimums.

Version rationale

This patch restores import of Ack-exported schemas containing pattern without changing the public Dart API. It also accepts standard propertyNames constraints; the core changelog records the fix and addition separately.

Verification on 92bab423

• Local structural checks: all six manifests and changelogs agree on 1.6.2; git diff --check passes.
• CI: formatting, workspace analysis and tests, and Chrome checks passed.
• Documentation passed.
• Release preflight passed: six-package API compatibility against published 1.6.1 baselines, both minimum SDK checks, deterministic generation, publish dry run, and Draft-7 batch validation.

Publication after review and merge

Use the exact reviewed release merge commit for v1.6.2. The tag starts the release workflow, which validates and publishes in dependency order. Confirm all six 1.6.2 versions on pub.dev, then create the GitHub Release.