feat(plugin): expose session forks with history filters

[rekram1-node]

Summary

Expose ctx.session.fork in both Effect and Promise plugins, retaining the existing fork inputs and result while adding an optional synchronous history callback:

const fork = await ctx.session.fork({
  sessionID,
  boundary: { type: "through" },
  filter: (messages) => messages.filter((message) => message.type !== "synthetic"),
})

• The callback receives decoded, settled Session messages in chronological order within the requested boundary. It can select or transform the messages and return an empty list.
• Core runs the callback once and records its resulting history in the durable fork event. Projection assigns new message identities and reserves ordering; replay uses the recorded result without invoking plugin code.
• Promise plugins retain normal client input/output conversion; their callback receives decoded Session messages, consistent with session hooks.
• Omitting the callback retains the existing fork path. This creates an ordinary independent fork with the existing fork lineage and instruction inheritance.
• Regenerate client/OpenAPI artifacts for the optional fork-event history payload.

Verification

• packages/core: bun test test/session-create.test.ts — 45 passing, including filtered history, boundary selection, replay, empty selection, subsequent prompt ordering, and Promise plugin integration
• packages/schema: bun test test/event-manifest.test.ts — 7 passing
• bun typecheck in packages/core, packages/plugin, packages/schema, and packages/client
• bun run generate in packages/client and packages/protocol
• git diff --check

[rekram1-node]

Split the generator fix and its tests into standalone PR #48024 against v2. This PR no longer changes httpapi-codegen; its generated client output matches the current v2 generator. After #48024 merges, regenerating this branch will preserve the named ProviderState references.