cardano-rpc: ReadTip

[carbolymer]

Context

Part of the node kernel access work (ADR-019) and the Kupo/Ogmios replacement effort (#1229): implements the SyncService.ReadTip method (#1218).

ReadTip returns the current chain tip as a fully populated BlockRef: slot, block header hash, block height and the slot timestamp in milliseconds. The handler reads the tip header straight from ChainDB through node kernel access (getTipHeader - no block body fetch), and derives the timestamp from era history the same way FetchBlock does. When the chain is at origin the tip field is left unset; when the node kernel has not initialised yet the call fails with UNAVAILABLE.

Implementation notes:

• Cardano.Api.Consensus gains getTipHeader, blockHash and blockSlot re-exports.
• A TraceRpcReadTipSpan trace is emitted around the handler; the node-side rendering and the rpc.request.SyncService.ReadTip prometheus counter land with the E2E test in gRPC: fetchBlock method cardano-node#6579.
• README coverage table: ReadTip flipped to supported.

Stacked on #1258; only the last commit is new here.

How to trust this PR

The handler is a ChainDB read plus field projection, so there are no unit tests - it needs a live ChainDB. E2E coverage lives in IntersectMBO/cardano-node#6579's Cardano.Testnet.Test.Rpc.FetchBlock: it calls ReadTip over gRPC against a running testnet and asserts the tip's slot and height are bounded below by a CLI query tip snapshot, the hash is 32 bytes, and the timestamp agrees with the slot-derived time within 1s.

Run it with:

TASTY_PATTERN='/RPC FetchBlock/' cabal test cardano-testnet-test

Checklist

• Commit sequence broadly makes sense and commits have useful messages
• New tests are added if needed and existing tests are updated. See Running tests for more details
• Self-reviewed the diff
• Changelog fragment added in .changes/