feat(mcp): bound tool-result passthrough and surface resource audience annotations

Route CallToolResult content through the render.rs content policy per
plans/mcp-resources-prompts-design.md §6 (T8): oversized text sliced at
TEXT_MAX_BYTES_CLAMP with a self-explaining truncation note, image/audio/
embedded blob content spilled (or inlined when UTF-8-clean) instead of
shipping base64 into model context, and structuredContent subject to the
same ceiling. Clamp server-controlled uri/mime metadata strings to the new
METADATA_MAX_BYTES bound in both the read and tool-result paths, sanitize
the terminal rendering of MCP dispatch errors while keeping raw text in
the tool_call_error payload, and surface resource audience annotations in
both mcp_search results and mcp_read metadata via the catalog.

src/mcp/mod.rs

@@ -125,6 +125,8 @@ pub struct CatalogItem {
     pub size: Option<u64>,
     #[serde(skip_serializing_if = "Option::is_none")]
     pub arguments: Option<Vec<PromptArgument>>,
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub audience: Option<Vec<String>>,
 }
 
 #[derive(Debug, Clone, Deserialize, Serialize)]