fix(core): improve clarity and actionable context in error messages

[Ahmed-Ali-313]

Link to Issue or Description of Change

Problem:
Several core error messages in runners.py, llm_agent.py, llm_request.py, and llm_agent_config.py were vague, missing the actual input values/types, or lacked guidance on how developers could fix the configuration issue.

Solution:

1. Updated Runner._resolve_app to output actual provided arguments/types.
2. Enhanced LlmAgent.set_default_model and set_default_live_model to report type(model).__name__.
3. Added actionable suggestions to LlmAgent.validate_generate_content_config explaining where to place tools, instruction, output_schema, and http_options.
4. Included actual types in LlmRequest.append_instructions and values in LlmAgentConfig._validate_model_sources.
5. Created unit tests in test_improved_error_messages.py.

Testing Plan

Unit Tests:

• I have added or updated unit tests for my change (tests/unittests/agents/test_improved_error_messages.py).
• All unit tests pass locally.

Pytest Summary:
13 passed in 4.64s

Manual End-to-End (E2E) Tests:
Verified that triggering misconfigurations (e.g., passing invalid model types or duplicate Runner arguments) now prints rich, actionable error tracebacks.

Checklist

• I have read the CONTRIBUTING.md document.
• I have performed a self-review of my own code.
• I have commented my code, particularly in hard-to-understand areas.
• I have added tests that prove my fix is effective or that my feature works.
• New and existing unit tests pass locally with my changes.
• I have manually tested my changes end-to-end.