feat: add RESEND_API_KEY environment variable support

[drish]

• Constructor now falls back to RESEND_API_KEY env var when ApiToken is not set.
• New ResendClient.Create() no-arg factory reads the env var and throws InvalidOperationException if absent.

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Summary by cubic

ResendClient now falls back to the RESEND_API_KEY environment variable when ApiToken isn’t set. Also adds a no-arg ResendClient.Create() that reads the env var and throws if missing.

• New Features
  • Constructor uses RESEND_API_KEY if ApiToken is empty.
  • Added ResendClient.Create() that reads RESEND_API_KEY; throws InvalidOperationException when absent.
  • Tests cover env fallback, no-arg success, and error case.

^{Written for commit 6b0e17a. Summary will update on new commits.}

[cubic-dev-ai]

No issues found across 2 files

Confidence score: 5/5

• Automated review surfaced no issues in the provided summaries.
• No files require special attention.

_{Auto-approved: Adds standard environment variable fallback for API keys and a helper factory method. Low-risk change with comprehensive test coverage.}