feat: propagate external_model_message upwards on errors

[kalvinnchau]

• Attempt to propagate external_model_message.message when we get a 400 Bad Request from databricks provider

ex:

( O)> hi
◓  Evolving ethereal entities...                                                                                                                                          
  2025-02-19T18:48:21.799301Z ERROR goose::agents::truncate: Error: Request failed: Request failed with status: 400 Bad Request. Message: Unknown error
    at crates/goose/src/agents/truncate.rs:279

becomes:

  2025-02-19T18:53:32.354126Z ERROR goose::agents::truncate: Error: Request failed: Request failed with status: 400 Bad Request. Message: Malformed input request: #: subject must not be valid against schema {"required":["messages"]}#/tools/4/name: string [6kr06od8__add nums] does not match pattern ^[a-zA-Z0-9_-]{1,64}$#/tools/4: required key [type] not found#/tools/4: extraneous key [input_schema] is not permitted#/tools/4: extraneous key [description] is not permitted#/tools/4/name: #/tools/4/name: 6kr06od8__add nums is not a valid enum value#/tools/4: required key [type] not found#/tools/4: required key [display_height_px] not found#/tools/4: required key [display_width_px] not found#/tools/4: extraneous key [input_schema] is not permitted#/tools/4: extraneous key [description] is not permitted#/tools/4/name: #/tools/4/name: 6kr06od8__add nums is not a valid enum value#/tools/4: required key [type] not found#/tools/4: extraneous key [input_schema] is not permitted#/tools/4: extraneous key [description] is not permitted#/tools/4/name: 6kr06od8__add nums is not a valid enum value#/tools/4/name: , please reformat your input and try again.
    at crates/goose/src/agents/truncate.rs:279