Allow array variables in mail> operator parameters

[i10416]

Summary

• Allow embedding array variables in to, cc, and bcc parameters of the mail> operator
• Users can now use expressions like to: ${address_list} where address_list is an array

Problem

When users write YAML like this:

_export:
  address_list: ["aaa@example.com", "bbb@example.com"]
+send_message:
  mail>:
  to: ${address_list}

The template engine replaces ${address_list} with its JSON representation as a string: to: '["aaa@example.com", "bbb@example.com"]'

The old code using getList() expected an already-parsed array and failed on JSON strings.

Solution

Changed to use parseList() and parseListOrGetEmpty() which:

1. Check if the value is a textual JSON string
2. Parse the JSON string if needed
3. Fall back to regular list parsing if not a string

This approach is already used by the for_each> operator for the same purpose.

Changes

• Changed getList() to parseList() for the to parameter
• Changed getListOrEmpty() to parseListOrGetEmpty() for cc and bcc parameters

Test Plan

• Added sendToWithArrayVariable() test - verifies JSON string parsing for to parameter
• Added sendCcWithArrayVariable() test - verifies JSON string parsing for cc parameter
• Added sendBccWithArrayVariable() test - verifies JSON string parsing for bcc parameter
• Tests simulate post-template-substitution state by setting values as JSON strings
• This matches the testing pattern in ConfigTest.verifyParseList()

Verification

The fix is verified by:

1. Existing Config.verifyParseList() test demonstrates parseList handles JSON strings
2. Updated tests correctly simulate the variable substitution behavior
3. Code analysis shows parseList calls tryParseNested which parses textual JSON