feat: support $expand=children on the driveItem endpoint

[dschmidt]

Implements $expand=children on GET /v1.0|v1beta1/drives/{drive-id}/items/{item-id}, the server side of opencloud-eu/libre-graph-api#68.

A folder listing currently needs two requests against graph (stat the folder, then list its children), where a single WebDAV PROPFIND with Depth: 1 returns both at once. With $expand=children clients get back to one request.

The children come from the same code path as the /children endpoint: the listing was moved into listDriveItemChildren and both callers share it, so the items and the error responses are identical. $expand is parsed by a helper mirroring the existing $select one. The relation stays unset for anything that is not a container.

No SDK bump needed, children is already part of the vendored driveItem model.

[codacy-production]

Up to standards ✅

🟢 Issues 0 issues

Results:
0 new issues

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🟢 Metrics 15 duplication

Metric	Results
Duplication	15

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🟢 Coverage 88.57% diff coverage · +0.05% coverage variation

Metric	Results
Coverage variation	✅ +0.05% coverage variation (-1.00%)
Diff coverage	✅ 88.57% diff coverage

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Coverage variation details

	Coverable lines	Covered lines	Coverage
Common ancestor commit (f60e2e5)	87708	20427	23.29%
Head commit (0750884)	87725 (+17)	20477 (+50)	23.34% (+0.05%)

Coverage variation is the difference between the coverage for the head and common ancestor commits of the pull request branch: <coverage of head commit> - <coverage of common ancestor commit>

Diff coverage details

	Coverable lines	Covered lines	Diff coverage
Pull request (#3445)	35	31	88.57%

Diff coverage is the percentage of lines that are covered by tests out of the coverable lines that the pull request added or modified: <covered lines added or modified>/<coverable lines added or modified> * 100%

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