[carbolymer]

Context

Every cardano-rpc method that carries a field_mask accepted it and ignored it.
This PR makes ReadParams and WatchMempool honour the mask, through one generic implementation driven by proto-lens descriptors.
There are no hand-written per-message tables, so each remaining method needs only a few lines to support masks.

Mask semantics:

• Paths start at the response message, for example values or tx.native_bytes.
• A path may continue through repeated fields and then applies to every element. This extends field_mask.proto, which allows a repeated field only as the last segment. The utxorpc Go SDK and Dolos both use such paths.
• Paths that do not name a field are ignored without an error. AIP-161 allows this for reads.
• An absent or empty mask, or a * path, returns everything.
• A mask whose paths are all ignored returns an empty response.

Laziness: proto-lens records have strict fields, so trimming a fully built response saves only bandwidth.
The handlers therefore skip the node queries and conversions that the mask does not need.
ReadParams skips the protocol parameters query and the era lookup when values is not selected, and the chain tip queries when ledger_tip is not selected.
WatchMempool serialises and converts a transaction only when the mask or the request predicate needs it.
Behaviour note: a ReadParams request that selects only ledger_tip no longer determines the era.

Not in this PR: ReadGenesis, ReadEraSummary, ReadUtxos, SearchUtxos, FetchBlock and FollowTip will follow in separate PRs.
ReadState, ReadData, ReadTx and DumpHistory are not implemented yet.

This PR is stacked on #1351 (the grpc-waitfortx branch), so the diff shows only the field mask commits.

References:

• https://github.com/protocolbuffers/protobuf/blob/main/java/util/src/main/java/com/google/protobuf/util/FieldMaskTree.java
• https://google.aip.dev/161
• https://github.com/protocolbuffers/protobuf/blob/main/src/google/protobuf/field_mask.proto

How to trust this PR

Review commit by commit. Each commit is one phase, and its tests were written first.

1. Add field mask path normalisation: a port of the normalisation in protobuf Java FieldMaskTree. Paths with empty segments are dropped.
2. Resolve field masks against proto descriptors: turns the normalised paths into a tree of known fields.
3. Trim responses to a field mask: oneof members are reset only when active, because proto-lens setters replace the whole oneof.
4. Add lazy field mask helpers: handlers describe each response field as a fill, and the mask decides which fills run.
5. Apply field masks in ReadParams.
6. Apply field masks in WatchMempool.
7. Changelog fragment.

Tests are unit tests and Hedgehog properties per phase.
The laziness tests pass error "forced" for the unselected inputs and fully evaluate the result.
The whole cardano-rpc-test suite passes.

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/