feat: track state-change block and canceler on IndexingAgreement

[MoonBoi9001]

Motivation

The IndexingAgreement entity currently captures the full business state of an agreement -- payer, indexer, pricing, timestamps of major transitions -- but two gaps make it awkward to consume as a source of truth for state reconciliation.

First, there is no block-indexed marker of the most recent state change. The existing lastUpdatedAt timestamp is only written in handleAgreementUpdated, so polling lastUpdatedAt_gt would miss accepts, cancels, and collections. Consumers that want a delta-style "what moved since block N" have to either iterate every entity on every poll or fall back to reading the event-log entities (which cuts against the aggregated-entity direction of PR #5).

Second, when an agreement is canceled the entity records state = CanceledByServiceProvider | CanceledByPayer but loses the actual signer address. Most cancels come directly from the payer or the indexer so the enum is sufficient, but in production an operator can sign on behalf of either party and the enum collapses that to just the party category. Dipper's chain_listener compares the canceler address to its own signer address to decide whether a cancel was self-initiated or not -- without the address on the entity, that check has to fall back to the event-log entity.

Summary

• Adds lastStateChangeBlock: BigInt! to IndexingAgreement. Every handler that transitions state stamps it with event.block.number: handleAgreementAccepted / Canceled / Updated / RCACollected on the RecurringCollector data source, and handleIndexingAgreementAccepted / Canceled / Updated on the SubgraphService data source. createOrLoadIndexingAgreement initialises it to zero.
• Adds canceledBy: Bytes! to IndexingAgreement, initialised to Bytes.empty() and written in a new handleIndexingAgreementCanceled handler that reads canceledOnBehalfOf from the SubgraphService event. The RecurringCollector handler continues to set the state enum; neither handler overwrites the other's field.
• Binds IndexingAgreementCanceled on the SubgraphService data source in subgraph.template.yaml.
• Matchstick tests: a new handleIndexingAgreementCanceled test that verifies canceledBy captures a distinct operator address (not inferred from payer/indexer) and that lastStateChangeBlock is stamped. Existing accept and update tests gain lastStateChangeBlock assertions.

Both new fields are additive and backwards-compatible: existing consumers that don't read them see no change, and new consumers that filter on lastStateChangeBlock_gt or read canceledBy get the delta-friendly shape they need.

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