refactor(session): remove message content mutation API

[rekram1-node]

Summary

Remove the completed-assistant-message content mutation introduced by #45015 for #44984.

• Remove PATCH /api/session/:sessionID/message/:messageID (session.messageUpdate), its generated client methods, and Core Session.updateMessage/session-handle operation and dedicated errors.
• Remove the replacement event from the public event surface and client live-state handling; regenerate clients and OpenAPI documents.
• Retain the existing durable event definition and projector solely to decode/replay records written by older releases. No database migration is required.
• Remove mutation-specific tests and adjust existing expectations; add no new tests.

Context and rationale

The original issue requested V1-style part deletion so clients could prune stored tool/reasoning payloads from long sessions, citing database growth. #45015 implemented a broader operation that replaced all content of a completed assistant message in an idle session.

Following maintainer discussion, arbitrary editing of persisted assistant history should not be part of the session API. Plugins that want to exclude tool/reasoning content from model input should use ctx.session.hook("context", ...) to transform the outgoing messages instead. The hook changes model context without rewriting stored history.

That distinction matters for the original report: context filtering does not reclaim disk space or remove content from stored/UI history. Storage retention is a separate concern; this PR does not claim that the context hook solves database growth.

Compatibility

This intentionally breaks callers of session.messageUpdate / Session.updateMessage and removes the corresponding public event. Existing session reads, prompts, execution, and other session operations remain available. Historical replacement events remain replayable, including for users who used the removed API before upgrading.

Created in a separate worktree from latest origin/v2 at d24f8b0810. Follow-up explanation on #44984 can happen after review.

Verification

• Existing Core session-owned and session-projector suites: 34 passing.
• Existing Server session-import and session-instances suites: 4 passing.
• Existing Client solid-data suite: 23 passing.
• Protocol and Schema event-manifest suites pass.
• Package typechecks pass: Core, Server, Client, Protocol, Schema, SDK, Plugin.
• Client and Protocol generated-output checks pass.
• Website typecheck, generated-output check, production build, and link validation pass.
• Formatting and git diff --check pass.

[pascalandr]

@rekram1-node Could you reconsider this removal and revert it until a narrower replacement is available?

OpenCode owns the database and session consistency, so it is precisely where safe cleanup should happen, not through direct SQL in downstream clients.

I think two different concerns are being conflated here: arbitrary history rewriting and selective removal of stored technical payloads. If messageUpdate is too permissive, could we constrain it or replace it with a dedicated prune operation, rather than remove the capability entirely?

An append-only history could record that content was pruned while preserving the necessary structure. Is there a concrete invariant that makes this unsafe, even for completed tool/reasoning blocks in idle sessions?

Given the immediate impact on CodeNomad 0.20.0, a temporary revert followed by a constrained replacement would let us address your concern without breaking adopted workflow in the meantime.