feat(plugin): support parent session creation - #52359

Summary

• Forward parentID through plugin and HTTP session.create, so children inherit the parent's Location rather than becoming root sessions. Missing parents return the declared 404.
• Regenerate the public clients and test the plugin, HTTP, and Promise-client paths.
• Replace feat(plugin): support parent session creation #47745 with a clean branch from current v2. The original work is credited to @dbpolito as a commit co-author; no MDX documentation edits are carried over.

Related issues

• Addresses item 3 only of [FEATURE]: Five session capabilities that exist in core but are unreachable from a plugin #49389; the other session API requests remain open.
• Related to [FEATURE]: Add explicit hidden or ephemeral sessions for plugin background tasks #40863's child-session use case. This does not add hidden or ephemeral session behavior.

Verification

• bun run generate in packages/client
• Plugin child-session test, HTTP create/import tests, and Promise-client serialization test pass.
• Scoped Core, Protocol, Server, Client, and Plugin typechecks pass.
• Full repository bun run check could not complete: the unrelated Posts package requires Node >=22.12.0; this host has Node 18.19.1.

Requested by: @rekram1-node (Aiden via Slack)