fix(chat): harden bridge_save/agentic_resume against empty and corrupt state

BLOCKER 1: use untyped reassignment (let x = ...) for the fallback bindings
in agentic_resume instead of re-declaring typed let bindings (let x: Type = ...)
for the same variable in the same scope. The typed form risks shadowing semantics
that differ from the established pattern used everywhere else in the loop
(e.g. agentic_loop line 720).

BLOCKER 2: add empty-string guards in both bridge_save and agentic_resume.
bridge_save now returns false without writing state if messages or tools_json
is empty — preventing syntactically invalid JSON blobs. agentic_resume now
returns an error envelope after the fallback resolution if either field is
still empty, rather than passing empty strings into agentic_loop which would
silently start a fresh turn with no context.

Also add tests:
- test_bridge_serialization.el: covers bridge_save empty-guard, golden-path
  raw-JSON round-trip, agentic_resume unknown/corrupt/missing-fields paths,
  and legacy string-escaped fallback path
- test_sessions_routes.el: covers DELETE and PATCH /api/sessions/:id routes
  (valid args, unknown id, empty body) and GET /api/sessions regression after
  removal of the duplicate route_sessions() handler

chat.el

@@ -766,6 +766,12 @@ fn agentic_loop(session_id: String, model: String, safe_sys: String, tools_json:
 // stored `messages` already includes the assistant turn that requested the tool, so
 // resume just appends the client's tool_result for `tool_use_id`.
 fn bridge_save(session_id: String, model: String, safe_sys: String, tools_json: String, messages: String, tools_log: String, tool_use_id: String) -> Bool {
+    // Guard: empty messages or tools_json would produce syntactically invalid JSON.
+    // Return false so the caller detects the failure rather than writing a corrupt
+    // blob that agentic_resume would later resume with no context.
+    if str_eq(messages, "") || str_eq(tools_json, "") {
+        return false
+    }
     // messages and tools_json are already well-formed JSON arrays; embed them as raw
     // JSON values (not string-escaped) so the round-trip through state_get/json_get_raw
     // never corrupts nested quotes. Scalar strings (model, safe_sys, tools_log,
@@ -796,9 +802,15 @@ fn agentic_resume(session_id: String, tool_use_id: String, content: String) -> S
     // messages_raw and tools_raw are embedded as raw JSON (not string-escaped);
     // fall back to legacy string-escaped fields for sessions saved before this fix.
     let messages: String = json_get_raw(blob, "messages_raw")
-    let messages: String = if str_eq(messages, "") { json_get(blob, "messages") } else { messages }
+    let messages = if str_eq(messages, "") { json_get(blob, "messages") } else { messages }
     let tools_json: String = json_get_raw(blob, "tools_raw")
-    let tools_json: String = if str_eq(tools_json, "") { json_get(blob, "tools_json") } else { tools_json }
+    let tools_json = if str_eq(tools_json, "") { json_get(blob, "tools_json") } else { tools_json }
+    // Guard: a corrupt or missing bridge blob (e.g. state cleared mid-flight)
+    // yields empty messages/tools. Return an error envelope rather than resuming
+    // with no context, which would cause the model to start a fresh turn.
+    if str_eq(messages, "") || str_eq(tools_json, "") {
+        return "{\"error\":\"corrupt bridge state\",\"reply\":\"\"}"
+    }
     let tools_log: String = json_get(blob, "tools_log")
     let saved_use_id: String = json_get(blob, "tool_use_id")