fix: let copied credentials replace inherited static auth

[pragnyanramtha]

Summary

• stop copy(credentials=...), copy(config=...), and copy(profile=...) from carrying over an inherited api_key or auth_token unless the caller explicitly passes a static credential again
• keep explicit static credentials shadowing copied/inherited credential providers unchanged
• add sync and async regression coverage for copy(credentials=...) from a statically-authenticated client

Root cause

Anthropic.copy() and AsyncAnthropic.copy() always passed api_key=api_key or self.api_key and auth_token=auth_token or self.auth_token into the new client. When a caller tried to replace a parent API key client with a credentials provider, the copied client still sent the parent X-Api-Key, so AccessTokenAuth short-circuited and the requested provider was silently disabled.

Validation

• uv run --frozen pytest tests/lib/test_credentials.py -q -n0 -k "copy_with_credentials_replaces_inherited_api_key or copy_with_explicit_api_key_shadows_inherited_credentials or copy_propagates_credentials"
• uv run --frozen pytest tests/lib/test_credentials.py -q -n0
• uv run --frozen pytest tests/test_client.py -q -n0 -k "copy"
• uv run --frozen ruff check src/anthropic/_client.py tests/lib/test_credentials.py
• uv run --frozen ruff format --check src/anthropic/_client.py tests/lib/test_credentials.py
• git diff --check

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