fuzz: Rework rpc fuzz target #35860

The rpc fuzz target constructs a vector of string args and passes that to RPCConvertValues.

This has many issues:

• Each of those strings could represent an array itself. E.g. via range argument or via ConsumeArrayRPCArgument. However, those strings may not be converted to an array via RPCConvertValues and just be passed on as string argument. Having a call to ConsumeArrayRPCArgument that ends up with a plain json string is confusing.
• The strings could only represent an object or json null, when a raw string represented such a serialized json and was also converted to one via RPCConvertValues. Having a call to ConsumeScalarRPCArgument that was intended to give a raw string but ends up with a arbitrary json object is confusing.

Fix those "stringly-typed" issues by making the fuzz target "type safe":

• Rename ConsumeScalarRPCArgument to ConsumeBasicRPCArgument and return a proper UniValue from it.
• The "consume string" case inside that function, which had a "double meaning" is turned into two type-safe cases: One that returns a json string and one that reads an arbitrary json from a string.
• A new case for json null is added.
• ConsumeRPCArgument is changed to cover both json arrays and json dicts properly.
• Pass the resulting positional UniValue array directly to the RPC method, avoiding the need for RPCConvertValues.

Making the fuzz target "type safe" is also the first step in making it schema-aware.

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Code Coverage & Benchmarks

For details see: https://corecheck.dev/bitcoin/bitcoin/pulls/35860.

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🚧 At least one of the CI tasks failed. _{Task tidy: https://github.com/bitcoin/bitcoin/actions/runs/30698617249/job/91365600298} _{LLM reason (✨ experimental): CI failed because clang-tidy reported misc-no-recursion errors (recursive call chain in test/fuzz/rpc.cpp, treated as warnings-as-errors).}

<details><summary>Hints</summary>

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• Possibly due to a silent merge conflict (the changes in this pull request being incompatible with the current code in the target branch). If so, make sure to rebase on the latest commit of the target branch.

• A sanitizer issue, which can only be found by compiling with the sanitizer and running the affected test.

• An intermittent issue.

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