feat(alerter): fail alertrule linting if rule exposes multiple ambiguous reserved words

[matucker-msft]

• Added AlertRule query result validation for duplicate reserved alert columns with different casing, such as Severity and severity.
• Reserved alert columns are tracked as a lowercase set and validated before row conversion so lint and execution fail clearly instead of silently choosing one value.
• Preserved support for non-reserved/custom columns with different casing.
• Updated linter error handling so lint failures include the underlying validation message.

[Copilot]

Pull request overview

This PR strengthens AlertRule linting/execution by detecting and rejecting ambiguous duplicate reserved alert output columns that differ only by casing (e.g., Severity vs severity), so adx-mon fails clearly instead of implicitly picking one value.

Changes:

• Added reserved-column duplicate detection (case-insensitive) before row-to-alert conversion in the alerter executor.
• Refactored Lint to be testable via an injectable Kusto client factory and to return a more informative lint error (lint.Err()).
• Added unit tests covering duplicate reserved columns (failure) and duplicate custom columns (allowed).

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
alerter/service.go	Refactors linting to use an injectable Kusto client factory and returns lint.Err() on failures.
alerter/lint_test.go	Adds a lint-level test ensuring duplicate reserved columns (different casing) fail lint with a clear message.
alerter/fake.go	Enhances lint handler to accumulate failure messages and expose them via Err().
alerter/engine/executor.go	Adds reserved notification column validation to reject duplicate reserved columns with different casing.
alerter/engine/executor_test.go	Adds executor tests for duplicate reserved columns (fail) and duplicate custom columns (allowed).