feat(plugin): support parent session creation

[dbpolito]

Issue for this PR

n/a

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

Allows plugins and API clients to pass parentID to session.create.

The server forwards the parent ID to Core, where the existing child-session behavior links the sessions and inherits the parent's Location. Missing parents now return the existing session-not-found response.

The generated clients and plugin docs have been updated.

How did you verify your code works?

• Ran typechecks for Core, Protocol, Server, Client, and Plugin
• Ran the Core session creation tests
• Regenerated the clients
• Built and typechecked the documentation site
• Checked generated OpenAPI output

Screenshots / recordings

Not applicable.

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

[opencode-agent]

Superseded by #52359, rebased as a clean change on current v2 with the parentID behavior retained and tests added. Thanks @dbpolito for the original implementation; credited as a commit co-author in the replacement. The MDX edits were intentionally omitted.