refactor(data): extract shared guardedApiCall for API-backed repositories

[netqo]

Summary

NewsRepositoryImpl and GeminiAssistantRepository each repeated the same guard sequence around their remote call: short-circuit to a failed Result when the API key is blank (logging the reason), then wrap the call in runCatching and log any failure under the repo tag. This PR extracts that sequence into a single inline fun guardedApiCall(...) in data/ApiCall.kt and routes both repositories through it.

Rationale

The two repositories had drifted into the same boilerplate. Centralizing it removes the duplication (DRY) and gives any future API-backed repository the same fail-fast-on-blank-key plus logged-failure behavior without re-implementing it. The helper is inline so the block lambda can still call the repositories' own suspend functions.

Behavior is preserved: blank keys still fail without touching the network, and exceptions still collapse to Result.failure with a full stack trace in logcat. Only the blank-key log wording changed slightly.

Verification

• ./gradlew ktlintCheck detekt testDebugUnitTest -> BUILD SUCCESSFUL
• ./gradlew assembleDebug -> BUILD SUCCESSFUL

Test plan

• ktlint + detekt clean
• JVM unit tests green
• Debug APK assembles

Checklist

• Commit messages follow Conventional Commits (see CONTRIBUTING.md).
• Branch name follows the naming convention (refactor/).
• The change is scoped: no unrelated edits sneaked into this PR.
• If user-visible behavior changed, CHANGELOG.md has been updated under [Unreleased]. (No user-visible change.)
• If a new dependency was added, it is justified in the PR description. (No new dependency.)