feat(spec): driveItem creation under drives/items/children + recursive paths

[dschmidt]

Summary

• Generalize the existing POST /v1beta1/drives/{drive-id}/root/children description from share-mounting only to the full MS Graph create-driveItem semantics (folder / file / remoteItem).
• Add a sibling POST /v1beta1/drives/{drive-id}/items/{item-id}/children operation (operationId: CreateChildDriveItem) for creating items under a non-root parent.
• Add @libre.graph.conflictBehavior query parameter (enum fail | replace) on both endpoints, controlling same-name child collisions. MS Graph's third value rename is omitted.
• Document the MS Graph colon-syntax URL forms (/root:/{path}:/children and /items/{item-id}:/{path}:/children) in both operation descriptions. OpenAPI cannot express the colon-delimited path segment, so these URL forms aren't represented as separate operations.
• Add @libre.graph.missingParentsBehavior query parameter (enum fail | create) as a libregraph extension controlling recursive creation of missing intermediate folders along a colon-syntax URL.

Notes

@libre.graph.conflictBehavior is a URL query parameter, not a body property. Following MS Graph's driveItem create docs: "The parameter @microsoft.graph.conflictBehavior should be included in the URL instead of the body of the request."

MS Graph is inconsistent on this across operations — its createUploadSession places the same annotation inside the request body (under item). This spec matches each operation's own MS Graph convention; the inconsistency is MS Graph's.

rename is omitted from the conflictBehavior enum. MS Graph defines fail | replace | rename. This spec exposes fail | replace only. The vendored reva exposes an overwrite primitive (net.ParseOverwrite, used in ocdav/{move,copy,put}.go) that maps naturally to replace. There is no equivalent helper for "pick a free name like foo (2).txt" — rename would require new server logic, not just wiring. Easier to add later than to spec a capability with no underlying primitive.

@libre.graph.missingParentsBehavior is a libregraph extension. No MS Graph equivalent. Controls whether the server auto-creates missing intermediate folders along a colon-syntax URL path. Default is fail (preserving current behavior); create opts into recursive creation.