PYR1-1799 Omit empty GWC extents from tile-layer PUTs

[danielhvs]

Purpose

Stop GeoSync from sending an empty <extent> in GWC tile-layer PUTs. On dev, fire-detections_active-fires and fire-detections_firis-perimeters failed registration on every run with a GWC 500, so the 1x1 metatile from #102 never landed and those layers stayed at metaWidthHeight [4, 4].

Cause

A tile layer that GeoServer creates itself has grid subsets with only a gridSetName, which means full gridset coverage. Live on geoserver-dev:

"gridSubsets": [{"gridSetName": "EPSG:4326"}, {"gridSetName": "EPSG:900913"}]

Both PUT builders copied those subsets back and always wrote <extent><coords></coords></extent>. GWC then fails to hash the empty bounding box:

java.lang.ArrayIndexOutOfBoundsException: Index 0 out of bounds for length 0
    at org.geowebcache.grid.BoundingBox.hashCode(BoundingBox.java:267)
    at org.geowebcache.config.XMLGridSubset.hashCode(XMLGridSubset.java:157)

Changes

1. update-cached-layer and update-cached-layer-metatiling write <extent> only when the subset has coords
2. Tests cover both builders with subsets that have no extent, and check that coords are still written when present

update-cached-layer is the raster TIME-filter path, and it had this bug before #102. It is behind the utility fire-weather-forecast_ecmwf_* registration failures on dev, which also appear before #102 was deployed.

Verified on dev

Both dev GeoSync UberJARs were built from 0a6bdc2, and I ran the syncs by hand:

1. sync-fire-detections on geoserver-dev: 0 registration failures, down from 2 on 3af114a (active-fires, firis-perimeters)
2. All 12 fire-detections_{active-fires,firis-perimeters,fire-history} tile layers now report metaWidthHeight [1, 1] and keep both grid subsets
3. geoserver01 log: 0 ArrayIndexOutOfBoundsException since the jar swap
4. sync-fire-weather-forecast on geoserver-utility-dev: fire-weather-forecast_ecmwf_20260924_00 returns status 0, Workspace updated. It failed 4 times earlier today

Related Issues

Closes PYR1-1799
Relates to PYR1-1786