Indexer: impose pagination if response is too big

[altafan]

Based on some config env var, the indexer should return paginated responses when the list of items is bigger than the threshold. Clietns must be aware of this, therefore requires coordination with sdks to introduce the feature

[arkanaai]

Codebase Context

Pagination infrastructure already exists but is currently opt-in — the gap is making it enforced when results exceed a configurable threshold.

Key files:

• internal/core/application/indexer.go — paginate[T]() helper (line 1065); hardcoded per-method max sizes (lines 30–35)
• internal/interface/grpc/handlers/indexer.go — gRPC handler layer, passes *Page params to service
• api-spec/protobuf/ark/v1/indexer.proto — IndexerPageRequest / IndexerPageResponse already defined
• internal/config/config.go — existing INDEXER_* env vars
• pkg/client-lib/indexer/grpc/client.go — paginatedFetch[T]() (line 744), client already handles pagination; maxPageSize = 1000, maxPages = 100

Current behaviour: if a client sends no page param, paginate() returns nil check → all items returned unbounded.

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Acceptance Criteria

• New env var (e.g. INDEXER_MAX_PAGE_SIZE) sets the per-response item cap; defaults to current per-method constants when unset
• When a request omits pagination params and the result set exceeds the cap, the server rejects with a descriptive error (or automatically applies page 1 with the cap) rather than returning an oversized payload
• All list endpoints respect the cap: GetVtxos, GetVtxoTree, GetVtxoTreeLeaves, GetForfeitTxs, GetConnectors, GetVtxoChain, GetVirtualTxs
• IndexerPageResponse is always populated in the response when the cap is active (clients can detect total > 1 to know more pages exist)
• SDK (pkg/client-lib/indexer/grpc/client.go) — paginatedFetch already handles multi-page fetching; verify maxPageSize client constant aligns with server cap or is configurable
• Integration test: request without pagination on a large dataset returns an error / first page; subsequent pages are fetchable
• Docs/changelog note on the breaking behaviour for clients that assumed unbounded responses

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Suggested Approach

1. Add MaxPageSize int32 to Config in internal/config/config.go, read from INDEXER_MAX_PAGE_SIZE (default 0 = use existing per-method constants)
2. Thread the config value into IndexerService at construction
3. In paginate[T](), if params == nil and len(items) > maxSize, either:
   • Return an errResponseTooLarge with a hint to paginate, or
   • Force params = &Page{PageSize: maxSize, PageNum: 1} and return page 1 (friendlier but client may not realise data is truncated — prefer the error path)
4. SDK: paginatedFetch already does the right thing; ensure the client-side maxPageSize constant doesn't exceed the server cap