Spring-ai refactor: move PgVectorStore to dedicated package and update builder …

…pattern

• Move PgVectorStore and related classes to org.springframework.ai.pg.vectorstore package
• Update builder pattern to use more idiomatic method names (e.g. withSchemaName -> schemaName)
• Deprecate existing constructors and old Builder class in favor of new static builder() method
• Update tests to reflect the new builder style usage

Comment From: markpollack

The postgresql library for vector store support uses com.pgvector so instead of org.springframework.ai.pg.vectorstore using org.springframework.ai.pgvector.vectorstore would be a better naming and is consistent with boot configuration property prefix.

Comment From: markpollack

updated reference docs and javadoc. change to use pgvector in package name.

Merged in 677a18e3d4e54139181898e23ae8beb1cf2ad3ed