fix: create new slices in the first configured library

[lihbr]

Summary

• Use the first slice library from prismic.config.json when creating slices via pull, sync, and slice create
• Fall back to the framework default library when no libraries are configured
• Add integration tests and update test helpers/matchers for custom library paths

Test plan

• Run npm test -- test/pull.test.ts test/slice-create.test.ts with E2E credentials
• Configure libraries: ["./slices/blog"] in a project, create a slice in Type Builder, run prismic pull, and confirm the slice lands under ./slices/blog/
• Run a subsequent prismic push and confirm the slice is recognized

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Note

Low Risk
Behavior change is limited to slice creation target paths; existing multi-library read paths were already in place, with new tests covering the configured-library case.

Overview
New slices from pull, sync, and slice create are written to the first path in prismic.config.json libraries, instead of always using the framework default (e.g. ./slices/).

Adapter.createSlice now defaults library to (await getSliceLibraries())[0], which already resolves configured libraries or falls back to the default when none are set.

Test helpers and toContainSlice were updated to scan all configured libraries (via getSliceLibraries / glob), and integration tests assert slices land under the first library (e.g. ./slices/blog) and not the root ./slices/ path.

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[jomifepe]

LGTM 💯

[angeloashmore]

I agree, it looks good. I made a few refactors for the sake of code cleanliness, but the functionality remains the same: #200