feat: verify DIPs proposal manifest network is supported

[MoonBoi9001]

TL;DR

Makes the indexer check a DIPs proposal's network against its configured DIPs networks at the manifest step, and treats a manifest with an empty network field as malformed. Small hardening change, stacked on #1041.

Motivation

When the indexer receives a DIPs proposal it fetches the subgraph manifest and reads the network the subgraph indexes. Today it never checks that network against the operator's configured DIPs networks directly — an unconfigured network is only caught later and indirectly, when the price lookup happens to find no minimum for it, and a manifest whose network field is empty slips through that gap entirely.

This adds an explicit check at the manifest step: a network the operator hasn't configured for DIPs is rejected up front with the unsupported-network reason, and an empty network field is reported as a malformed manifest rather than masquerading as an unsupported network.

Summary

• Check the manifest's network against the indexer's configured DIPs networks at the manifest step.
• Reject a manifest with an empty network field as malformed, not as an unsupported network.
• A genuinely unsupported network behaves as before, just caught earlier and explicitly.
• A malformed manifest still maps to the generic "other" reason on the wire (no new wire code here).
• Add a unit test covering the empty-network case.
• Stacked on feat: classify DIPs proposal rejections by area and nature #1041; rebase onto main-dips-rebased once that merges.