Enforce configurable maximum ledger transaction size

[Copilot]

Ledger entries store transaction body size in a 6-byte field, but CCF did not enforce a configurable bound before allocation. Corrupt or oversized entries can now be rejected with clear errors, and oversized user transactions do not prevent later transactions from processing.

• Configuration
  • Added ledger.max_transaction_size, defaulting to 100MB.
  • Exposed the setting in host config schema and test infrastructure.

"ledger": {
  "chunk_size": "5MB",
  "max_transaction_size": "100MB"
}

• Enforcement

  • Applies to the serialized transaction body size stored in the ledger entry header.
  • Excludes the fixed 8-byte ledger entry header.
  • Includes ledger encryption header, public domain size field, public domain, and encrypted private domain.
  • Checked on serialization, deserialization, snapshot handling, and ledger recovery entry extraction.

• Failure behavior

  • Oversized local transactions roll back KV mutations and return 413 Payload Too Large.
  • Oversized deserialized/recovered entries fail with explicit size-limit errors rather than large allocations.

• Coverage and docs

  • Added KV serialization/deserialization tests for the cap.
  • Added e2e coverage confirming a rejected oversized transaction does not stop subsequent transactions.
  • Documented the new configuration limit in the changelog and generated config schema.