docs(openrpc): add method-level examples to generated schema

[Minhcardanian]

Overview

Adds method-level OpenRPC examples to the generated schema so OpenRPC Playground / Inspector can prefill requests and display concrete examples for all emitted methods.

This change updates the OpenRPC schema generator in node/src/openrpc.rs, regenerates docs/openrpc.json, and adds a regression test asserting that every emitted method includes a non-empty examples array with params and result.

No runtime behavior, RPC semantics, method inventory, or versioning changes are included.

🗹 TODO before merging

• Ready

📌 Submission Checklist

• Changes are backward-compatible (or flagged if breaking)
• Pull request description explains why the change is needed
• Self-reviewed the diff
• I have included a change file, or skipped for this reason: skipped because this change only affects generated OpenRPC documentation/schema output and related generator/test coverage; no runtime/user-facing behavior change
• If the changes introduce a new feature, I have bumped the node minor version
• Update documentation (if relevant)
• Updated AGENTS.md if build commands, architecture, or workflows changed
• No new todos introduced

🧪 Testing Evidence

Please describe any additional testing aside from CI:

• Additional tests are provided (if possible)

Additional validation performed locally:

• cargo fmt --check -p midnight-node
• python3 -m json.tool docs/openrpc.json >/tmp/openrpc.valid
• git diff --check -- docs/openrpc.json node/src/openrpc.rs

Additionally verified:

• method count: 68
• methods missing examples: 0

Local environment limitation:

• cargo test -p midnight-node --lib openrpc::tests could not complete because protoc was unavailable
• nix develop . -c cargo test -p midnight-node --lib openrpc::tests got past protoc but failed in the midnight-node-runtime build script because libstdc++.so.6 was not available in the dynamic linker path

🔱 Fork Strategy

• Node Runtime Update
• Node Client Update
• Other: documentation/schema generator update only
• N/A

Links

[CLAassistant]

All committers have signed the CLA.