Session extension: pass the prompt language to the context section and make it async #526

Goal

The session extension's context section can follow the session's prompt language and can read the extension's own (async) data store. Today the hook gets neither, so an edition that keeps per-server, per-group or per-tag data in its extension tables cannot build a language-correct context section without blocking or duplicating core logic.

Context

• SessionExtension::context_section(&self, _scope: &ServerScope) -> Option<String> is synchronous and receives only the scope (crates/app-logic/src/session_extension.rs:32).
• Its only caller, append_extension_section (crates/app-logic/src/session_extension.rs:140, call at :149), already has the prompt language in parts.language (used for the heading via extension_heading, :128, :156) but does not pass it on.
• append_extension_section is called from two async contexts:
  • crates/app-shell/src/commands/connect.rs:500 (session connect, inside an async fn that already awaits extension_scope),
  • crates/app-shell/src/commands/chat.rs:276 (rebuilt on every user message, inside an async loop that already awaits extension_scope).
• The language type is PromptLanguage (crates/app-logic/src/system_prompt.rs:135: De, En, Other(ResponseLanguage)).
• Extension-owned tables (Spec 0038, section 2a) are reached through an async database handle, so a synchronous hook can only read them by blocking the runtime or keeping a separate cache.
• Precedent for an async edition hook: OutgoingTransform::prepare_outgoing uses #[async_trait] (crates/core/src/outgoing.rs:84-93); async-trait is already a dependency of app-logic (crates/app-logic/Cargo.toml:27).
• Governing docs: Spec 0038, section 2 "Sitzungs-Erweiterung" (docs/specs/0038-repository-app-shell-ci.md:38-68), ADR 0136 (docs/adr/0136-session-extension-point.md).

Requirements

• SessionExtension::context_section receives the prompt language of the session as a PromptLanguage argument, the same value append_extension_section uses for the section heading.
• context_section becomes async (via #[async_trait] on SessionExtension, like OutgoingTransform), so an extension can await reads from its own data store. No block_in_place or other blocking bridge in the core.
• The default implementation still returns None.
• append_extension_section becomes async; both call sites (connect.rs:500, chat.rs:276) await it. The order stays: core paragraphs, then the extension heading, then the redacted and fenced section (redact, then fence; ADR 0034).
• known_secrets, server_removed and group_removed keep their current signatures and behaviour.
• Without a registered extension the system prompt and the redactor stay byte-identical to before.

Acceptance Criteria

• context_section is an async trait method that takes &ServerScope and PromptLanguage and returns Option<String>; its default returns None.
• append_extension_section passes parts.language to context_section; a test extension that returns a different text per language yields the German text for PromptLanguage::De and the English text for PromptLanguage::En (counter-proof: the test fails if the language is not forwarded).
• A test extension whose context_section awaits (e.g. a tokio::sync read) works through append_extension_section without blocking the runtime.
• The existing session_extension tests (fence after core text in both languages, empty/whitespace section adds nothing, no extension adds nothing, section is redacted before fencing) pass in their async form.
• Both call sites in connect.rs and chat.rs await the async section; behaviour for a session without an extension is unchanged.
• Spec 0038 (section 2, "Sitzungs-Erweiterung") states that the context section receives the prompt language and may be answered asynchronously; ADR 0136 gets a short note on the async hook and the language argument.
• Relevant automated tests pass

Technical Notes

• Affected: crates/app-logic/src/session_extension.rs (trait, append_extension_section, tests incl. TestExtension at :248 and RecordingExtension), crates/app-shell/src/commands/connect.rs, crates/app-shell/src/commands/chat.rs, and every other impl SessionExtension in tests (crates/app-logic/src/mcp_lookup.rs:252, crates/app-logic/src/session.rs:1869); they only implement known_secrets but the trait gains #[async_trait], so check they still compile.
• This changes an internal extension-point API; editions that implement SessionExtension have to adapt their context_section to the new signature.
• Security: the section remains untrusted content. Redact-then-fence, the server_note fence, and setting untrusted_content_ingested must stay exactly as today. Filter, risk classification and redaction rules are not touched.
• No data model or migration change.

Spec impact: update 0038

Related Work

• Extension point: session context section and per-server known secrets for redaction #463 (introduced the session extension point)
• Outgoing transform: pass the server's group chain like the session extension #501 (group chain in the scope of the outgoing transform)