Define provider fixtures and mocks

[Will-thom]

Summary

• add docs/provider-fixtures-and-mocks.md
• define provider fixture principles, suggested layout, fake client behavior, error scenarios, and test layers
• document that default tests remain credential-free and network-free
• link the contract from testing docs, provider context plan, and backlog

Validation

• git diff --check
• go test ./...

Closes #85

[github-actions]

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