fix(session-export): distinguish unsupported raw artifacts

SessionPersistence.readRaw previously used undefined for two unrelated states: a supported backend could not find the requested session, or the backend had no per-session artifact concept at all. The export endpoint consequently reported an existing SQLite-backed session as HTTP 404, which falsely diagnosed storage capability as session absence.

Make raw-artifact support an explicit backend capability. Unsupported backends now fail their inherited readRaw path loudly and the host answers 501 before reading, while undefined retains the single meaning of an absent artifact on a supporting backend. First-party backends, test providers, generated API catalogs, bilingual persistence docs, and export error contracts now state that distinction; focused tests cover both the 501 and the inherited rejection.

docs/subsystems/persistence.zh.md

@@ -124,7 +124,7 @@ interface CreateSessionOptions {
 
 ## `SessionRawArtifact`——逐字存储工件文本
 
-后端为单个会话自持的工件文本，与其持久化写入的字节逐字一致（按物理编码解码）。`readRaw` 返回它而不从解析后事件重建，因此后端特定的序列化（chunk 打包、键序、换行）得以保留；没有每会话工件的后端（如 SQLite）继承 `undefined` 默认。
+后端为单个会话自持的工件文本，与其持久化写入的字节逐字一致（按物理编码解码）。`readRaw` 返回它而不从解析后事件重建，因此后端特定的序列化（chunk 打包、键序、换行）得以保留。Consumer 须先检查 `supportsRawArtifacts`：`false` 表示后端不提供此能力（如 SQLite），而 `readRaw(...) === undefined` 表示受支持的后端没有该会话的已实体化工件。
 
 ```ts type-equiv
 /** A backend's own raw artifact text for one session, verbatim. */
@@ -262,13 +262,15 @@ abstract locate(meta: SessionHeader): SessionLocation | undefined
  * bytes the backend wrote (decoded from its physical encoding, e.g. a
  * decompressed JSONL). The returned `content` is the raw text, not a
  * reconstruction from parsed events, so it preserves backend-specific
- * serialization (chunk packing, key order, line breaks). Backends without a
- * per-session artifact (SQLite) inherit the `undefined` default.
+ * serialization (chunk packing, key order, line breaks). Callers first test
+ * {@link supportsRawArtifacts}; `undefined` then means only that the requested
+ * session has no materialized artifact.
  * @param _id - the persisted session to read (unused by the default: no
  * per-session artifact).
  * @param signal - optional cancellation for backend read work.
  * @returns the raw artifact plus its parsed header, or `undefined` when the
- * session is absent or the backend owns no per-session artifact.
+ * session is absent.
+ * @throws when this backend does not expose per-session raw artifacts.
  */
 readRaw(_id: SessionId, signal?: AbortSignal): Promise<SessionRawArtifact | undefined>