fix(code-mode): use MCP structuredContent for proper JS object access

[codefromthecrypt]

Summary

Tool results with outputSchema (e.g. { content: string }) were returned as plain text. JS code like result.content returned undefined because the MCP structuredContent field was ignored.

• Prioritize structuredContent from MCP response when available
• Extract parse_result_to_js helper for JSON→JsValue coercion
• Show output schema structure in signatures (not just object)
• Add lightweight tests proving structured vs plain text behavior

Type of Change

• Feature
• Bug fix
• Refactor / Code quality
• Performance improvement
• Documentation
• Tests
• Security fix
• Build / Release
• Other (specify below)

AI Assistance

• This PR was created or reviewed with AI assistance

Testing

After this ONE SHOT even with a crappy model, it knew that the call had an structured output field

$ cargo run -p goose-cli --release -- run --text "Search for get_file_info and write_file tools. Use them to save info about $PWD to $PWD/result.txt." --with-builtin code_execution --with-extension "npx -y @modelcontextprotocol/server-filesystem $PWD"
   Compiling goose v1.17.0 (/Users/codefromthecrypt/oss/goose/crates/goose)
   Compiling goose-bench v1.17.0 (/Users/codefromthecrypt/oss/goose/crates/goose-bench)
   Compiling goose-cli v1.17.0 (/Users/codefromthecrypt/oss/goose/crates/goose-cli)
    Finished `release` profile [optimized] target(s) in 52.12s
     Running `target/release/goose run --text 'Search for get_file_info and write_file tools. Use them to save info about /Users/codefromthecrypt/oss/goose to /Users/codefromthecrypt/oss/goose/result.txt.' --with-builtin code_execution --with-extension 'npx -y @modelcontextprotocol/server-filesystem /Users/codefromthecrypt/oss/goose'`
starting session | provider: tetrate model: gpt-5-nano
    session id: 20251219_24
    working directory: /Users/codefromthecrypt/oss/goose

─── read_module | code_execution ──────────────────────────
module_path: goose_b051fheg


─── execute_code | code_execution ──────────────────────────
code: import { get_file_info, write_file } from "goose_b051fheg";
                                                                                                                                                                                                                          
const sourcePath = "/Users/codefromthecrypt/oss/goose";                                                                                                                                                                   
const destPath = "/Users/codefromthecrypt/oss/goose/result.txt";                                                                                                                                                          
                                                                                                                                                                                                                          
const info = get_file_info...                                                                                                                                                                                             

I have saved the file information to /Users/codefromthecrypt/oss/goose/result.txt.

Details:
- Source path queried: /Users/codefromthecrypt/oss/goose
- Destination: /Users/codefromthecrypt/oss/goose/result.txt
- Operation: Retrieved file metadata and wrote it to the destination

If you want the metadata in a different format (JSON, YAML, etc.) or want additional fields, tell me and I can adjust.

Related Issues

Relates to #6188

[codefromthecrypt]

these are the coolest tests

[Copilot]

Pull request overview

This PR fixes a bug where MCP tool results with outputSchema were returned as plain text instead of structured JSON objects, causing JavaScript property access (e.g., result.content) to fail.

Key Changes:

• Prioritizes structured_content from MCP responses over plain text content
• Extracts parse_result_to_js helper to consistently coerce tool results into proper JavaScript values
• Enhances type signature generation to display detailed object/array structures instead of generic "object" labels

[michaelneale]

very nice, and looking at live tests for code mode:

Provider: openrouter
Model: google/gemini-2.5-pro

starting session | provider: openrouter model: google/gemini-2.5-pro
    session id: 20251219_1
    working directory: /tmp/tmp.M5q6b81WPx

─── execute_code | code_execution ──────────────────────────
code: import { shell } from "developer";
shell({ command: "ls -la" });


```text
total 20
drwx------  2 runner runner  4096 Dec 19 07:44 .
drwxrwxrwt 15 root   root   12288 Dec 19 07:44 ..
-rw-r--r--  1 runner runner     6 Dec 19 07:44 hello.txt

✓ SUCCESS: Test passed - code_execution tool called

first go! Much improved.