t8n should use the transaction rlp provided by the testing framwork

[gurukamath]

Transactions are the only part of the t8n which use some sort of a json round trip workaround in the PR. In order to avoid this workaround (and fully make use of the rlp that the testing framework generates) we will need to make some small changes on the specs side, particularly wrt the decode_transaction function. I aim to tackle this in a subsequent PR.

Originally posted by @gurukamath in #2924

[gurukamath]

Replacing the JSON round-trip with self.fork.decode_transaction(tx.rlp())
requires three coupled changes left to a follow-up PR:

• Pre-Berlin forks (Frontier → Muir Glacier) have no decode_transaction — they predate typed txs. Needs either a spec backport adding the function to all 9 forks, or a has_decode_transaction fallback in T8N (which also has to handle the pre-Berlin Transaction vs Berlin+ LegacyTransaction rename).
• Berlin → Prague's decode_transaction rejects raw legacy RLP bytes — Amsterdam codified EIP-2718's tx[0] >= 0xC0 → LegacyTransaction branch but pre-Amsterdam forks raise TransactionTypeError. Needs a WET spec backport across 14 forks.
• Exception mapper has no DecodingError entry — the contract-creating-blob-tx case currently flows through TransactionLoad → check_transaction → TransactionTypeContractCreationError. Switching to decode_transaction(tx.rlp()) makes RLP raise DecodingError before check_transaction runs, so the mapper (or the affected tests' expected exceptions) needs updating.

The first two are spec changes updates; the third is tightly coupled to them landing first.