Normalize missing MCP transport collection fields for ACP compatibility

[CsBigDataHub]

Error I have been getting

((:object (jsonrpc . "2.0") (id . 4)
  (error (code . -32602) (message . "Invalid params")
         (data . "data did not match any variant of untagged enum McpServer at line 1 column 118")))
 (:json
  . "{\"jsonrpc\":\"2.0\",\"id\":4,\"error\":{\"code\":-32602,\"message\":\"Invalid params\",\"data\":\"data did not match any variant of untagged enum McpServer at line 1 column 118\"}}"))

my config -

(setq agent-shell-mcp-servers
        '(((name . "context7-mcp")
          (command . "context7-mcp"))
         ((name . "splunk-nonprod")
          (command . "/Users/mymac/GitRepos/mcp-scripts/splunk-mcp.py")
          (env . (((name . "SPLUNK_HOST") (value . "stage.splunk.com")))))
         ((name . "splunk-prod")
          (command . "/Users/mymac/GitRepos/mcp-scripts/splunk-mcp.py")
          (env . (((name . "SPLUNK_HOST") (value . "prod.splunk.com")))))))

This change makes MCP server normalization more robust when users provide minimal hand-written configs.

agent-shell--mcp-servers now fills in the collection fields ACP expects based on transport type before normalizing list values into vectors:

• stdio-style transports now default missing args and env to empty vectors
• HTTP/SSE transports now default missing headers to an empty vector

This prevents downstream consumers from receiving incomplete payloads when optional collection fields are omitted from user config.

Checklist

• I agree to communicate (PR description and comments) with the author myself (not AI-generated).
• I've reviewed all code in PR myself and will vouch for its quality.
• I've read and followed the Contributing guidelines.
• I've filed a feature request/discussion for a new feature.
• I've added tests where applicable.
• I've updated documentation where necessary.
• I've run M-x checkdoc and M-x byte-compile-file.