fix: execution listing queries should be faster now

[Copilot]

Pull request description

Rewrote postgres execution listing queries to use text[] instead of jsonb arrays for tag/label/selector filtering, eliminating ~18s timeouts when listing executions for an organization. Added comprehensive unit tests covering all changed query paths and the new text[]-based filter encoding.

Checklist (choose whats happened)

• breaking change! (describe)
• tested locally
• tested on cluster
• added new dependencies
• updated the docs
• added a test

Breaking changes

Changes

• Query rewrite: GetTestWorkflowExecutions, GetTestWorkflowExecutionsSummary, GetFinishedTestWorkflowExecutions, CountTestWorkflowExecutions, GetTestWorkflowExecutionsTotals — replaced jsonb array params for tags/labels/selectors with text[] using key, key=value, and key:not_exists encoding
• Param builders: buildTestWorkflowExecutionParams / buildTestWorkflowExecutionTotalParams updated; new helpers parseSelectorToText, parseLabelSelectorToText, parseTagSelectorToText produce the text[] slices
• sqlc tests (executions.sql_test.go): replaced permissive ExpectQuery("SELECT") with regexp.QuoteMeta(<query constant>); added tests for CountTestWorkflowExecutions, GetFinishedTestWorkflowExecutions, GetTestWorkflowExecutions, GetTestWorkflowExecutionsSummary
• Param builder tests (postgres_test.go): added TestBuildTestWorkflowExecutionParams, TestBuildTestWorkflowExecutionTotalParams, TestParseSelectorToText, TestParseTagSelectorToText, TestParseLabelSelectorToText covering key-only, key=value, multi-value same key, not_exists, whitespace trimming, and mixed conditions

Fixes

• Old queries timed out (~18s for listing the executions of an organization)

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