fix(eth): return null for replaced transaction lookups

[rvagg]

Replaces #13661

When you ask for a specific transaction hash, we only return the data for that specific transaction hash, not a replacement transaction that will have a different hash but has replaced it in practice. Standard Filecoin APIs encourage viewing replaced messages and what they are replacing as roughly the same. This PR takes the opinion that Ethereum APIs shouldn't silently convert for you if the replacement got through and you're asking for the original.

DIscussion in slack; there's one concern that this makes it more difficult for Curio to find replacement msgs but I'm not sure if that's structural or can be easily solved by just querying by the replacement hash.

[Copilot]

Pull request overview

This PR adjusts Lotus’ Ethereum-compatible transaction lookup semantics so that eth_getTransactionByHash and eth_getTransactionReceipt return null for transactions that were replaced (RBF) before execution, instead of silently returning data for the replacement transaction. This aligns Ethereum API behavior with expected replacement semantics while still supporting Filecoin’s “replacement-aware” behavior via the allowReplaced flag.

Changes:

• Add a sentinel error (stmgr.ErrMessageReplaced) to classify “message was replaced” outcomes from message searches.
• Update StateSearchMsg(..., allowReplaced=false) to treat “replaced” as “not found” (return nil, nil) and update ETH transaction/receipt lookups to use allowReplaced=false.
• Add an integration test covering replacement lookups and update the changelog.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
node/impl/full/state.go	Converts ErrMessageReplaced into a nil result when allowReplaced=false for StateSearchMsg.
node/impl/eth/transaction.go	Makes ETH hash/receipt lookups return null for replaced tx hashes by disabling replacement-following.
chain/stmgr/searchwait.go	Introduces ErrMessageReplaced and wraps relevant replacement-detection errors with it.
chain/stmgr/searchwait_test.go	Asserts replacement failures are classified via errors.Is(..., ErrMessageReplaced).
itests/eth_hash_lookup_test.go	Adds an end-to-end test ensuring replaced tx hash lookups return null.
CHANGELOG.md	Documents the ETH API behavior change.

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