feat: add mailto SMTP support and parity coverage

[elibosley]

Summary by CodeRabbit

Release Notes

• New Features

  • Added email notification support via mailto: and mailtos: URL schemes, enabling SMTP-based message delivery with configurable encryption modes, authentication credentials, and multiple recipient fields (to, cc, bcc, reply-to).

• Tests

  • Added comprehensive test coverage including parity validation between implementations to ensure consistency and reliability of email notifications.

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[coderabbitai]

📝 Walkthrough

Walkthrough

This pull request adds mailto and mailtos URL scheme support for SMTP-based email notifications. It includes a complete SMTP client implementation with TLS/encryption support, schema registration, test infrastructure for SMTP message capture, and comprehensive parity testing against Python Apprise behavior.

Changes

Cohort / File(s)	Summary
Core Notification Implementation internal/cli/cli.go, internal/notify/mailto_target.go, internal/notify/registry_email.go	Adds mailto/mailtos handler in CLI dispatcher, implements MailtoTarget SMTP client with URL parsing, TLS modes (insecure/STARTTLS/SSL), PLAIN authentication, MIME message construction with quoted-printable encoding, and per-recipient delivery. Registers both schemes in email schema map.
Test Infrastructure internal/testutil/smtp_capture.go, internal/testutil/scripts/capture_smtp.py	Introduces lightweight TCP-based SMTP server for test message capture with thread-safe access, and Python CLI wrapper for Apprise SMTP testing to enable cross-language parity validation.
Parity Testing internal/parity/mailto_parity_test.go, internal/parity/non_http_schemas.go	Adds comprehensive parity test comparing Go mailto sender against Python Apprise SMTP behavior, including header parsing, content-transfer-encoding decoding, and message normalization. Provides helper for identifying non-HTTPS schemas.
Test Adjustments internal/parity/e2e_parity_test.go, internal/parity/e2e_schema_exercise_test.go, internal/parity/provider_coverage_test.go	Adds guards to skip non-HTTPS schemas in three existing test suites, excluding mailto/mailtos from HTTPS-only test coverage.

Sequence Diagram

sequenceDiagram
    participant Client
    participant MailtoTarget
    participant SMTPConn as SMTP Connection
    participant SMTPServer as SMTP Server

    Client->>MailtoTarget: Send(body, title)
    MailtoTarget->>MailtoTarget: Parse mailto URL
    MailtoTarget->>MailtoTarget: Extract host, port, credentials<br/>from, to, cc, bcc, reply-to
    MailtoTarget->>SMTPConn: Dial SMTP server
    SMTPConn->>SMTPServer: TCP connect
    SMTPServer-->>SMTPConn: 220 SMTP ready
    
    MailtoTarget->>SMTPConn: Send EHLO
    SMTPServer-->>SMTPConn: 250 capabilities
    
    alt Use STARTTLS or SSL
        MailtoTarget->>SMTPConn: Upgrade TLS
        SMTPConn->>SMTPServer: STARTTLS/SSL handshake
        SMTPServer-->>SMTPConn: TLS established
    end
    
    alt Credentials provided
        MailtoTarget->>SMTPConn: AUTH PLAIN
        SMTPServer-->>SMTPConn: 235 authenticated
    end
    
    loop For each recipient
        MailtoTarget->>MailtoTarget: Build MIME message<br/>(subject, from, to, headers, body)
        MailtoTarget->>SMTPConn: MAIL FROM
        MailtoTarget->>SMTPConn: RCPT TO
        MailtoTarget->>SMTPConn: DATA + message payload
        SMTPServer-->>SMTPConn: 250 Message queued
    end
    
    MailtoTarget->>SMTPConn: QUIT
    SMTPConn->>SMTPServer: Close connection

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Estimated code review effort

🎯 4 (Complex) | ⏱️ ~60 minutes

Poem

🐰 A rabbit's hop through SMTP's gate,

With mailto schemes configured just right,

TLS secured, credentials slate,

MIME-compliant messages take flight! ✉️

Parity tested from Python to Go,

This notification path steals the show! 🚀

🚥 Pre-merge checks | ✅ 2 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (2 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly and concisely describes the main change: adding mailto SMTP support with parity test coverage.

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✨ Finishing touches

• 📝 Generate docstrings

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment
• Commit unit tests in branch feat/mailto-support

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