feat: Add comprehensive SMTP test suite for Deno

- Implemented SMTP client utilities in `test/helpers/smtp.client.ts` for creating test clients, sending emails, and testing connections.
- Developed SMTP protocol test utilities in `test/helpers/utils.ts` for managing TCP connections, sending commands, and handling responses.
- Created a detailed README in `test/readme.md` outlining the test framework, infrastructure, organization, and running instructions.
- Ported CMD-01: EHLO Command tests in `test/suite/smtpserver_commands/test.cmd-01.ehlo-command.test.ts` with multiple scenarios including valid and invalid hostnames.
- Ported CMD-02: MAIL FROM Command tests in `test/suite/smtpserver_commands/test.cmd-02.mail-from.test.ts` covering valid address acceptance, invalid address rejection, SIZE parameter support, and command sequence enforcement.

test/fixtures/test-cert.pem

@@ -0,0 +1,21 @@
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test/fixtures/test-key.pem

@@ -0,0 +1,27 @@
+-----BEGIN RSA PRIVATE KEY-----
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