Support partial user email search

[PANiXiDA]

What changed

• Changed the user email filter from exact equality to a case-insensitive PostgreSQL ILIKE substring search.
• Standardized ILIKE calls on two named arguments with a $%{value.Trim()}% pattern.
• Removed the redundant explicit null guard around ILIKE match expressions.
• Aligned UserReadDbModel.Email nullability with the existing NOT NULL database column and removed the obsolete explicit Mapperly mapping and null suppression for Email.
• Removed full email-address validation from the list query while retaining pagination validation.
• Added architecture conventions that enforce the agreed ILIKE call shape and reject explicit null guards for its match expression.
• Updated unit, integration, and functional test scenarios to use partial mixed-case email filters.

Why

The list endpoint accepts an email search fragment, not a complete email address. The previous domain email validation rejected valid search fragments with HTTP 400, while the repository required an exact case-sensitive match.

PostgreSQL already excludes a row when a nullable expression evaluates to NULL ILIKE pattern, so an explicit null guard is redundant. In this schema, identity.asp_net_users.email is already NOT NULL; only the read-model annotation incorrectly described it as nullable.

Verification

• GitHub CI run: passed.
• All 10 test projects passed.
• Architecture tests: 110 passed, 0 failed, 0 skipped.
• Formatting, Helm validation, aggregated test reports, coverage generation, and the coverage threshold passed.
• Local Release build of Identity.Infrastructure and its dependencies passed with 0 warnings and 0 errors.
• Local tests were not run by request.

Compatibility

• Migrations: none; the email column is already NOT NULL.
• Contract changes: no request or response shape changes; the existing email filter now accepts substrings and matches case-insensitively.
• Configuration changes: none.
• Breaking changes: none.