test: expand GitHub datasource E2E coverage

[akkikumar72]

• Add variable-query editor coverage that verifies the serialized Releases request and rendered preview values
• Add representative Branches coverage for filter serialization and rendered query results
• Keep the scope bounded to deterministic mocks instead of duplicating all query modes
• Checks: Playwright 5/5, Jest 21/21, typecheck, lint, production build, Go tests, and Prettier on Node 24.19.0

Closes #205

[Copilot]

Pull request overview

Expands end-to-end coverage for the GitHub datasource query/variable editors using deterministic mocked query responses, addressing the gap noted in #205.

Changes:

• Adds a new E2E test for the Variable Query Editor to validate Releases query serialization (metricFindQuery) and rendered preview values.
• Extends Query Editor E2E coverage with a Branches test validating filter serialization and rendered table output.
• Introduces additional mock responses to support the new E2E scenarios (variable refId and branches frame).

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
tests/VariableQueryEditor.spec.ts	New E2E test covering variable query (Releases) request serialization and preview rendering.
tests/QueryEditor.spec.ts	Adds E2E coverage for Branches query filter serialization and table rendering.
tests/mocks/github-response.ts	Adds deterministic mock responses for variable queries (metricFindQuery) and branches results.

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