refactor(juno): restrict felt.Felt to accept only 0x prefixed hex values

[MaksymMalicki]

Previously, Felt accepted decimal numbers, binary strings, and bare hex. Now it only accepts 0x-prefixed hexadecimal strings. This tightens input validation for all JSON-deserialized felt values.

[codecov]

Codecov Report

❌ Patch coverage is 80.00000% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 75.41%. Comparing base (afe8c38) to head (1c63412).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
core/felt/felt.go	50.00%	0 Missing and 2 partials ⚠️
starknet/class.go	81.81%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3488      +/-   ##
==========================================
- Coverage   75.52%   75.41%   -0.11%     
==========================================
  Files         384      384              
  Lines       34937    34968      +31     
==========================================
- Hits        26386    26372      -14     
- Misses       6693     6696       +3     
- Partials     1858     1900      +42     

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[thiagodeev]

I have one question: if our concern is only about not allowing users to set decimals on felt fields values that should only allow hex values in RPC requests, why then not make this change specific to the RPC?
I mean, we are applying a general change in our entire code base (since felt is used everywhere (consensus, p2p, cryptography...)), where a felt from a decimal is not a problem at all (in some cases, it's even a required behavior (like the EntryPointOffset situation). IMO, this is not good. We should have a patch specifically for RPC-related code.

One idea is to create a new felt type, specifically to be used in all RPC requests, and implement the filter logic on it. Then, we update all RPC methods to use this type instead of felt.Felt.

E.g:

type FeltInput struct {
  *felt.Felt // with it as an embedded field, we can access all felt methods out of the box from the FeltInput type
}

func (z *FeltInput) UnmarshalJSON(data []byte) error {
  // filter logic here....
}

It's just one idea. How it should be done is something that we need to discuss, but I really believe this is the right choice.

---

Edit: after internal discussion with the team, we decided to move forward with the proposed approach in the PR.