[codex] Fix graph subsystem correctness bugs

[somethingwithproof]

Summary

• Preserve the authenticated user when proxying graph image renders to a remote collector.
• Reject remote graph responses that do not contain the expected image marker before serving bytes as PNG data.
• Restore the remote collector temporary error handler with finally so failed collector calls do not leak handlers.
• Key the GPRINT consolidation-function cache by data source and template RRD id.
• Use returned realtime graph bytes when RRDtool succeeds but the realtime cache file is unavailable.
• Add a Docker-backed graph subsystem regression probe.

Open PR overlap

• Checked open PRs before preparing this branch.
• PR fix(rrd): correctness fixes in graph rendering and rrdtool output handling #7183 already fixes the GPRINT consolidation-function cache issue on 1.2.x only.
• PR chore(security): harden report actions and theme handling #7169 touches graph files on develop, but its graph changes are unrelated theme/remote-agent hardening and do not fix these graph correctness issues.

Validation

• php -l graph_image.php
• php -l graph_realtime.php
• php -l lib/functions.php
• php -l lib/rrd.php
• php -l tests/e2e/graph_subsystem_probe.php
• bash -n tests/e2e/graph_subsystem_docker.sh
• bash tests/e2e/graph_subsystem_docker.sh

The Docker probe passes. It emits existing PHP deprecation notices from Cacti/vendor code under the container PHP version before printing PASS graph subsystem docker probe.