Add table-driven tests for the filter parser

[khaylebfortune]

Cover query-param parsing with a table of inputs and expected filters.

Acceptance criteria

• Implement the change described above, scoped to the testing area.
• Add/extend tests covering the new behavior (table-driven where practical).
• No breaking changes to existing endpoints, config, or schema.
• go build ./..., go vet ./..., golangci-lint run, and go test ./... all pass.

Labels: medium, testing