Add seqera:// NIO filesystem for Seqera Platform datasets

[jorgee]

Summary

• Implements a seqera:// NIO FileSystemProvider in nf-tower, enabling Nextflow pipelines to reference Seqera Platform datasets as standard file paths (e.g. seqera://org/workspace/datasets/name)
• Path hierarchy: root → org → workspace → resource type → dataset file (with optional @version pinning)
• All HTTP traffic delegates to the existing TowerClient / HxClient so token refresh is shared
• Registers via META-INF/services/java.nio.file.spi.FileSystemProvider

New files

Package	Purpose
dataset/DatasetDto, DatasetVersionDto, WorkspaceOrgDto	API response DTOs
dataset/SeqeraDatasetClient	API calls: list orgs/workspaces/datasets/versions, download
fs/SeqeraPath	Path implementation with 0–4 depth hierarchy
fs/SeqeraFileSystem	FileSystem with lazy org/workspace/dataset caches
fs/SeqeraFileSystemProvider	FileSystemProvider SPI: read, list, attributes, copy
fs/SeqeraFileAttributes	BasicFileAttributes backed by dataset metadata
fs/SeqeraPathFactory	Nextflow PathFactory integration
fs/ResourceTypeHandler, fs/DatasetsResourceHandler	Extensibility interface for future resource types

Changes to existing files

• TowerClient: added public sendApiRequest() + GET support in makeRequest(); moved initHttpClient() out of constructor to TowerFactory
• TowerFactory: client() now also activates when accessToken is present (not only when enabled = true), so seqera:// paths work without explicitly setting tower.enabled
• TowerPlugin: registers SeqeraPathFactory
• FileHelper: recognises seqera:// scheme

Test plan

• SeqeraPathTest — path parsing, URI round-trips, relativize/resolve, getFileName, asUri
• SeqeraFileSystemTest — cache loading, workspace/dataset resolution, thread safety
• SeqeraFileSystemProviderTest — newInputStream (latest + pinned version), readAttributes, newDirectoryStream, error propagation
• SeqeraDatasetClientTest — API URL construction, response mapping, error handling

Run with:

./gradlew :plugins:nf-tower:test

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[jorgee]

Some comments about current implementation:

• Some refactoring is needed to decouple the Tower client (API calls) from the Observer. A client initialization must be done at filesystem initialization and another at observer. Due to token refresh, HxClient must be shared to avoid authentication problems when using different clients. I would like to make it after merging Add platform-related metadata in Lineage records #6545
• The Dataset API does not allow streaming the content, so read and write are done through temporary files.
• Only csv and tsv extensions are allowed; the format is recognized by the extension.
• Due to the above comments, I am considering making it read-only
• Every change in the dataset creates a new version seqera://org/workspace/datasets/name accesses the latest version and seqera://org/workspace/datasets/name@version