Add PostgreSQL session storage

[kongweiguang]

Summary

• Add a PostgreSQL-backed session storage extension.
• Register the module in extensions, BOM, and all-in-one distribution.
• Document the new PostgreSQL session dependency.

Validation

• mvn -pl agentscope-extensions/agentscope-extensions-session-postgresql spotless:apply test
• Real PostgreSQL smoke test against postgres:16-alpine in Docker
• mvn -pl agentscope-distribution/agentscope-bom -DskipTests package
• mvn -pl agentscope-distribution/agentscope-all -am -DskipTests package

[codecov]

Codecov Report

❌ Patch coverage is 96.16613% with 12 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
...scope/core/session/postgresql/PostgresSession.java	96.16%	4 Missing and 8 partials ⚠️

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