Merge bitcoin/bitcoin#30532: refactor: remove deprecated TxidFromString() in favour of transaction_identifier::FromHex()

f553e6d86fe114e90585ea6d4b8e345a209cae5d refactor: remove TxidFromString (stickies-v)
285ab50ace4c04209f331ccaf121152b977cc490 test: replace WtxidFromString with Wtxid::FromHex (stickies-v)
9a0b2a69c4d836d7382f6fe7703de40288fc7421 fuzz: increase FromHex() coverage (stickies-v)
526a87ba6b4f20592ad3c090b82e83ecff2107fc test: add uint256::FromHex unittest coverage (stickies-v)

Pull request description:

  Since https://github.com/bitcoin/bitcoin/pull/30482/commits/fab6ddbee64e50d5e2f499aebca35b5911896ec4, `TxidFromString()` has been deprecated because it is less robust than the `transaction_identifier::FromHex()` introduced in [the same PR](https://github.com/bitcoin/bitcoin/pull/30482). Specifically, it tries to recover from length-mismatches, recover from untrimmed whitespace, 0x-prefix and garbage at the end, instead of simply requiring exactly 64 hex-only characters.

  In this PR, `TxidFromString` is removed completely to clean up the code and prevent further unsafe usage. Unit and fuzz test coverage on `uint256::FromHex()` and functions that wrap it is increased.

  Note: `TxidFromSring` allowed users to prefix strings with "0x", this is no longer allowed for `transaction_identifier::FromHex()`, so a helper function for input validation may prove helpful in the future _(this overlaps with the `uint256::uint256S()` vs `uint256::FromHex()` future cleanup)_. It is not relevant to this PR, though, besides the fact that this unused (except for in tests) functionality is removed.

  The only users of `TxidFromString` are:
  - `test`, where it is straightforward to drop in the new `FromHex()` methods without much further concern
  - `qt` coincontrol. There is no need for input validation here, but txids are not guaranteed to be 64 characters. This is already handled by the existing code, so again, using `FromHex()` here seems quite straightforward.

  Addresses @maflcko's suggestion: https://github.com/bitcoin/bitcoin/pull/30482#discussion_r1691826934

  Also removes `WtxidFromString()`, which is a test-only helper function.

  ### Testing GUI changes

  To test the GUI coincontrol affected lines, `regtest` is probably the easiest way to quickly get some test coins, you can use e.g.

  ```
  alias cli="./src/bitcoin-cli -regtest"
  cli createwallet "coincontrol"
  # generate 10 spendable outputs on 1 address
  cli generatetoaddress 10 $(cli -rpcwallet=coincontrol getnewaddress)
  # generate 10 spendable outputs on another address
  cli generatetoaddress 10 $(cli -rpcwallet=coincontrol getnewaddress)
  # make previous outputs spendable
  cli generatetoaddress 100 $(cli -rpcwallet=coincontrol getnewaddress)
  ```

ACKs for top commit:
  maflcko:
    re-ACK f553e6d86fe114e90585ea6d4b8e345a209cae5d 🔻
  hodlinator:
    ACK f553e6d86fe114e90585ea6d4b8e345a209cae5d
  paplorinc:
    ACK f553e6d86fe114e90585ea6d4b8e345a209cae5d
  TheCharlatan:
    Nice, ACK f553e6d86fe114e90585ea6d4b8e345a209cae5d

Tree-SHA512: c1c7e6ea4cbf05cf660ba178ffc4f35f0328f7aa6ad81872e2462fb91a6a22e4681ff64b3d0202a5a9abcb650c939561585cd309164a69ab6081c0765ee271ef

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