fix(evm-rpc): tolerate per-tx null trace/stateDiff in trace_replayBlockTransactions

[elina-chertova]

Problem

The evm-dump for gnosis-mainnet is stuck in a fatal crash-loop:

DataValidationError: server returned unexpected result: invalid value at /0/stateDiff: null is not an object
  at evm/evm-rpc/lib/rpc.js  (getResultValidator for trace_replayBlockTransactions)
  rpcMethod: trace_replayBlockTransactions   params: [<hash>, ["trace","stateDiff"]]

The upstream provider returns a valid HTTP 200 for trace_replayBlockTransactions, but with stateDiff: null for an individual transaction (verified: the same gnosis block returns a proper stateDiff object from two other providers, so the null is a provider-side defect, not inherent to the chain). Because getTraceTransactionReplayValidator requires stateDiff (and trace) to be a non-null object when the tracer is requested, validation throws a fatal DataValidationError and the whole ingestion process crashes and crash-loops — a single provider hiccup takes down the dump.

#497 already added tolerance for a block-level null (whole array). This extends the same treatment to the per-transaction case it missed (/0/stateDiff).

Fix

• getTraceTransactionReplayValidator: make the per-tx trace and stateDiff fields nullable, so a transient null no longer fatally fails schema validation.
• addTraceTxReplays: when a requested tracer is null for any tx, flag the block _isInvalid for retry (mirroring the block-level null path from evm-rpc: tolerate null from trace_replayBlockTransactions (fixes tempo hotblocks crash-loop) #497 and the debug_* _isInvalid guards) instead of letting corrupt data through.

This makes the process survive a genuinely-external malformed response and retry the block, rather than crash-looping. (Complementary operator action — routing gnosis trace/stateDiff traffic away from the degraded provider — is being handled separately in ops; that only removes today's trigger, whereas this stops any provider hiccup from crash-looping ingestion.)

Test

evm/evm-rpc/test/trace-replay-null.test.ts — asserts the validator accepts a per-tx null stateDiff/trace. Fails on the pre-fix code ("... null is not an object"), passes after.

• rush build -t @subsquid/evm-rpc — green (compiles)
• vitest --run (evm-rpc) — 172 passed / 28 skipped, incl. the 2 new
• tsc --noEmit -p tsconfig.test.json — clean

Falsification: if a provider persistently returns null for a block, _isInvalid retries 5× then throws (same as every other invalid-block path) — the durable resolution to a persistently-degraded provider is the routing change, not this PR; this PR ensures a transient null can no longer crash ingestion.