Remove _remote.repositories from cache + use offline mode for forked PRs (#1391)

## Summary
Two fixes for forked PR dependency resolution:

1. **Warmer**: Remove `_remote.repositories` marker files before saving
the cache. These files track which repo ID (`jfrog-central`) each
artifact was downloaded from. When forked PRs restore the cache in
offline mode, Maven refuses artifacts because they were "not downloaded
from central".

2. **Fork composite action**: Use Maven offline mode (`-o` via
`.mvn/maven.config`) + empty `settings.xml`. With clean cache (no repo
ID markers), offline mode resolves everything from `~/.m2/repository`
with zero network requests.

Also deleted the existing stale cache entry to force a fresh save.

## Test plan
- [x] Verified locally: `mvn -o` with removed `_remote.repositories` →
BUILD SUCCESS, zero network requests
- [ ] After merge: re-trigger cache warmer, then re-run PR #1371 CI

NO_CHANGELOG=true

This pull request was AI-assisted by Isaac.

---------

Signed-off-by: Gopal Lal <gopal.lal@databricks.com>
Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

Author: gopalldb

.github/actions/setup-maven/action.yml

@@ -21,51 +21,34 @@ runs:
         key: maven-deps-${{ hashFiles('**/pom.xml') }}
         restore-keys: maven-deps-
 
-    - name: Configure Maven without credentials (fork)
+    - name: Configure Maven for cache-only resolution (fork)
       if: inputs.is-fork == 'true'
       shell: bash
       run: |
         mkdir -p ~/.m2
+
+        # Remove _remote.repositories marker files. These track which remote
+        # repo each artifact came from. Without them, Maven treats all cached
+        # artifacts as locally installed and resolves them without contacting
+        # any remote server.
+        find ~/.m2/repository -name '_remote.repositories' -delete 2>/dev/null || true
+        echo "Removed _remote.repositories markers from cache"
+
+        # Enable Maven offline mode via .mvn/maven.config. This file is read
+        # automatically by Maven as default CLI arguments. Offline mode prevents
+        # all network requests — no JFrog auth needed, no Maven Central fallback.
+        # Combined with _remote.repositories removal, Maven resolves everything
+        # from the local cache.
+        mkdir -p .mvn
+        echo "-o" >> .mvn/maven.config
+        echo "Maven offline mode enabled via .mvn/maven.config"
+
+        # Empty settings — no mirrors needed in offline mode.
         cat > ~/.m2/settings.xml << 'SETTINGS_EOF'
-        <settings>
-          <mirrors>
-            <mirror>
-              <id>jfrog-central</id>
-              <mirrorOf>*</mirrorOf>
-              <url>https://databricks.jfrog.io/artifactory/db-maven/</url>
-            </mirror>
-          </mirrors>
-          <!-- No <servers> block: forked PRs use cached dependencies only. -->
-          <!-- updatePolicy=never prevents Maven from checking the remote for
-               updates, avoiding 401 errors when artifacts are already cached. -->
-          <profiles>
-            <profile>
-              <id>cache-only</id>
-              <repositories>
-                <repository>
-                  <id>jfrog-central</id>
-                  <url>https://databricks.jfrog.io/artifactory/db-maven/</url>
-                  <releases><updatePolicy>never</updatePolicy></releases>
-                  <snapshots><updatePolicy>never</updatePolicy></snapshots>
-                </repository>
-              </repositories>
-              <pluginRepositories>
-                <pluginRepository>
-                  <id>jfrog-central</id>
-                  <url>https://databricks.jfrog.io/artifactory/db-maven/</url>
-                  <releases><updatePolicy>never</updatePolicy></releases>
-                  <snapshots><updatePolicy>never</updatePolicy></snapshots>
-                </pluginRepository>
-              </pluginRepositories>
-            </profile>
-          </profiles>
-          <activeProfiles>
-            <activeProfile>cache-only</activeProfile>
-          </activeProfiles>
-        </settings>
+        <settings />
         SETTINGS_EOF
 
-        echo "Maven configured for forked PR (cache-only, no JFrog credentials)"
+        echo "Maven configured for forked PR (cache-only, no remote access)"
 
     # --- Same-repo path: full JFrog OIDC authentication ---
     - name: Get JFrog OIDC token

.github/workflows/warmMavenCache.yml

@@ -160,6 +160,16 @@ jobs:
 
           echo "Dependency resolution complete"
 
+      - name: Remove _remote.repositories before saving cache
+        shell: bash
+        run: |
+          # Remove _remote.repositories marker files before saving. These track
+          # which remote repo ID each artifact was downloaded from (jfrog-central).
+          # Without them, forked PRs can use the cache in offline mode without
+          # Maven complaining about repo ID mismatches.
+          COUNT=$(find ~/.m2/repository -name '_remote.repositories' -delete -print | wc -l)
+          echo "Removed ${COUNT} _remote.repositories markers"
+
       - name: Save Maven dependency cache
         uses: actions/cache/save@0057852bfaa89a56745cba8c7296529d2fc39830 # v4
         with: