fix: isolate default component configurations

[ningyixi]

Problem

The default configuration for each of BaseTool, BaseResource, and BasePrompt is constructed once in the function signature. Instances that omit config therefore share a mutable object: setting first.config.title or .description also changes another instance's public name/description, including instances constructed later.

Changes

Use a None default and create a fresh base configuration per instance. Explicitly supplied configuration objects are retained by identity, so callers can still share a config deliberately. This changes two runtime lines in each base class; schemas and execution methods are unchanged.

Add public-behavior regression coverage for all three component types, checking existing and subsequently constructed instances, plus explicit-config identity checks. The regression test restores the original values in teardown so it also runs safely on the unfixed implementation.

Validation

Python 3.12.14, dependencies from uv sync:

• New tests on base cbf3e3e: 3 failed, 3 passed, with all failures caused by another instance's metadata changing.
• Base-component suite after the fix: 46 passed.
• env -u MINIMAX_API_KEY uv run pytest --cov=atomic_agents atomic-agents: 624 passed, 3 skipped; 1,875 of 1,884 statements covered (99.5%, rounded to 99% in the report). The three live MiniMax tests were skipped; no model requests were made.
• Full configured Black check (217 files), Flake8, and file-scoped pre-commit hooks pass.

AI assistance: prepared and verified with Codex.