Plugin metadata prefetch fails hard even when result is unused (pre-installed plugins, exact versions)

## Bug report ### Expected behavior and actual behavior When all requested plugins are pinned to exact versions and already installed in `$NXF_HOME/plugins/`, Nextflow should start them from the local directory without requiring network access. Instead, Nextflow unconditionally calls the plugin registry API at startup (`PluginsFacade.groovy:415`), and if the registry is unreachable, the run aborts with a `PluginRuntimeException` even though the fetched metadata is never used. This affects users in airlocked/firewalled environments who pre-install plugins into their container images. They have done everything right (pinned versions, pre-populated the plugins directory), but Nextflow still requires network access to a service whose response it will discard. ### Steps to reproduce the problem 1. Install a plugin locally: ```bash curl -L -o nf-schema-2.4.2.zip \ "https://registry.nextflow.io/api/v1/plugins/nf-schema/2.4.2/download/nf-schema-2.4.2.zip" unzip nf-schema-2.4.2.zip -d $NXF_HOME/plugins/ ``` 2. Create a workflow that pins the plugin version: ```groovy plugins { id 'nf-schema@2.4.2' } workflow { log.info "Plugin loaded successfully" } ``` 3. Block access to `registry.nextflow.io` (e.g. via firewall rule or `/etc/hosts`) and run: ```bash nextflow run main.nf ``` 4. Nextflow fails at the metadata prefetch, before it ever checks whether the plugin is already installed locally. Note: `NXF_OFFLINE=true` works around the plugin issue but also blocks remote pipeline cloning (`CmdRun.groovy:620-621`), making it unusable for environments that need to fetch pipelines from a Git server (e.g. Seqera Platform pulling from GitLab). ### Program output ``` Unable to connect to https://registry.nextflow.io/api/v1/plugins/dependencies?plugins=nf-schema%402.4.2&nextflowVersion=25.10.2 - cause: <connection error> ``` ### Environment * Nextflow version: 25.10.2+ (any version using the registry-based plugin system) * Java version: any * Operating system: any * Bash version: any ### Additional context The root cause is in the startup sequence in `PluginsFacade.start()`: ```groovy // prefetch the plugins meta updater.prefetchMetadata(startable) // <-- always makes HTTP call, throws on failure // finally start the plugins for( PluginRef plugin : startable ) { updater.prepareAndStart(plugin.id, plugin.version) // <-- would succeed from local dir } ``` In `PluginUpdater.load0()`, when a plugin has an exact version and is already installed, no metadata is consulted: - Line 351-359: version resolution is skipped (exact semver provided) - Line 372-375: directory exists, download is skipped - Line 386-406: transitive deps are read from the plugin's own MANIFEST, not registry metadata The prefetched metadata is write-only for this code path. A minimal fix would be to make the `prefetchMetadata` failure non-fatal (log a warning instead of throwing), so that the actual failure surfaces later only if something genuinely needs the metadata (version resolution for unpinned plugins, download URLs for missing plugins). If nothing needs it, the run succeeds. See also #6988, which describes a related problem with `NXF_OFFLINE` being too coarse-grained.