unify usage of JSON via Box<RawValue>

[niklasad1]

Builds on-top of #1521

This PR unifies that Box is used everywhere instead of having untypes JSON Strings with a the native Rust String type.

Some additional changes that I got annoyed/needed for this work properly:

• Rename StringError -> SubscriptionError
• SubscriptionError now supports both representing errors as Strings or JSON
• SubscriptionMessage, don't serialize internally let the user take care of that. This makes it easier to understand the API

[jsdw]

Mostly LGTM!

I wonder if we can avoid all of the double quoted strings by using serde_json::value::to_raw_value("some string") instead (I didn't comment every place this was done because there are quite a lot :D).

Also a small Q about the subscriptionerror!

[Copilot]

Copilot reviewed 30 out of 30 changed files in this pull request and generated no comments.

Comments suppressed due to low confidence (1)

core/src/server/subscription.rs:504

• Consider using structured JSON construction (e.g., the serde_json::json! macro) instead of string formatting to build JSON messages; this can help reduce potential formatting issues.

let json_str = format!("{\"jsonrpc\":\"2.0\",\"method\":\"{method}\",\"params\":{\"subscription\":{sub_id},\"result\":{result}}}");

[niklasad1]

This was actually faster than the manual stuff above

jsonrpsee_types_serialize_sub_manual
                        time:   [177.53 ns 177.85 ns 178.23 ns]
Found 3 outliers among 100 measurements (3.00%)
  3 (3.00%) high severe

jsonrpsee_types_serialize_sub_serde
                        time:   [90.732 ns 91.579 ns 92.648 ns]
Found 5 outliers among 100 measurements (5.00%)
  3 (3.00%) high mild
  2 (2.00%) high severe

[jsdw]

Nice! Liking the to_raw_values and such :D

[Copilot]

Copilot reviewed 31 out of 31 changed files in this pull request and generated 2 comments.

Comments suppressed due to low confidence (1)

core/src/server/method_response.rs:264

• [nitpick] Using RawValue::NULL directly can reduce code clarity. Consider defining a named constant for an empty JSON message to enhance maintainability and readability.

json: RawValue::NULL.to_owned(),