eth, core/state: self-heal missing contract code on stateless import

[lucca30]

Stacked on #2401 — base branch is lmartins/stateless-incomplete-state-guard,
not develop. Review/merge #2401 first; the diff here is only the three
commits on top of it. It builds directly on the typed error #2401 introduces.

Problem

#2401 makes a stateless node detect a missing contract code and reject the
block instead of masking it. But rejection alone means the node stalls on that
block until the code happens to arrive by some other path. A WIT2 witness never
carries code, so there is no in-band way to recover — the node needs to fetch
exactly the missing blob.

Fix

Three layers, bottom-up:

1. core/state — MissingCodeError{Addr, Hash} types the two "code is not
   found" paths in stateObject.Code/CodeSize (message unchanged, so
   witness-regen fixtures still match) and is wrapped into ExecuteStateless's
   result. A caller can now errors.As the missing hash, and tell a missing
   code (fetchable, content-addressed) from a missing trie node (an
   incomplete witness, not fetchable).

2. eth/protocols/snap — Syncer.FetchByteCodes fetches specific bytecodes
   from a connected snap peer outside the bulk sync loop, verifying each blob
   against its hash and failing over across peers. Responses route by a top-bit
   reqid (loop reqids are uint64(rand.Int63()), top bit clear, so the id
   spaces never collide). Peers are usable because Register/Unregister track
   them independently of a running Sync cycle — the stateless case, where the
   initial heal has already completed.

3. eth/downloader — on a *state.MissingCodeError from a stateless batch
   import, fetch that content-addressed blob, persist it to the chain db, and
   retry the batch. Bounded by maxStatelessCodeHeals so a blob no peer can
   serve degrades to the existing safe failure instead of looping; a batch
   referencing several missing codes heals them across successive attempts. Any
   non-code failure is left to the caller's existing path untouched.

Test

• core/state — the guard test (core: reject stateless blocks on incomplete state or code #2401) is extended to assert the hash
  round-trips via errors.As.
• eth/protocols/snap — TestFetchByteCodesOnDemand (well-behaving peer;
  unknown hash never fabricated) and TestFetchByteCodesFailsOverAndVerifies
  (fails over past a peer returning bytes that don't match the requested hash).
• eth/downloader — TestRecoverMissingStatelessCode (a servable miss is
  fetched, verified and persisted; a non-code error and an unservable blob are
  both left for the safe failure path, writing nothing).

Scope / risk

Every fetched blob is verified content-addressed (keccak(code) == hash) before
it is persisted, so a malicious peer cannot inject wrong code — it simply fails
over. Recovery is bounded and safe-stops on an unservable code. Together with
#2401 this recovers any missing code at execution time regardless of how the
gap arose (heal incompleteness, crash window, or fast-forward), which is why it
is deployed ahead of any mechanism-specific prevention.

[codecov]

Codecov Report

❌ Patch coverage is 66.40625% with 43 lines in your changes missing coverage. Please review.
⚠️ Please upload report for BASE (lmartins/stateless-incomplete-state-guard@7448950). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
eth/protocols/snap/ondemand.go	66.29%	23 Missing and 7 partials ⚠️
eth/downloader/bor_downloader.go	62.96%	9 Missing and 1 partial ⚠️
core/state/state_object.go	25.00%	3 Missing ⚠️

❌ Your patch check has failed because the patch coverage (66.40%) is below the target coverage (90.00%). You can increase the patch coverage or adjust the target coverage.

Additional details and impacted files

@@                             Coverage Diff                              @@
##             lmartins/stateless-incomplete-state-guard    #2402   +/-   ##
============================================================================
  Coverage                                             ?   55.60%           
============================================================================
  Files                                                ?      919           
  Lines                                                ?   167291           
  Branches                                             ?        0           
============================================================================
  Hits                                                 ?    93024           
  Misses                                               ?    68784           
  Partials                                             ?     5483           

Files with missing lines	Coverage Δ
core/stateless.go	78.18% <100.00%> (ø)
eth/protocols/snap/sync.go	76.43% <100.00%> (ø)
core/state/state_object.go	87.27% <25.00%> (ø)
eth/downloader/bor_downloader.go	64.96% <62.96%> (ø)
eth/protocols/snap/ondemand.go	66.29% <66.29%> (ø)

Files with missing lines	Coverage Δ
core/stateless.go	78.18% <100.00%> (ø)
eth/protocols/snap/sync.go	76.43% <100.00%> (ø)
core/state/state_object.go	87.27% <25.00%> (ø)
eth/downloader/bor_downloader.go	64.96% <62.96%> (ø)
eth/protocols/snap/ondemand.go	66.29% <66.29%> (ø)

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