Fix panic when updating proofs

[n8maninger]

Don't panic when attempting to update proofs and return an error instead.

[Copilot]

Pull Request Overview

This PR fixes a panic when updating element proofs by modifying the update flow in Manager and improving the test utility for proof updates. Key changes include the introduction of a non-panicking error handling mechanism in updateV2TransactionProofs, additions to test utilities to support proof updates without panicking, and updating error messages with ErrInvalidElementProof.

Reviewed Changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
testutil/proofs.go	Adds an in‑memory state store for testing proof updates; contains a goroutine that still panics on error.
chain/pool_test.go	Updates pool transaction tests to use the new element state store and verifies error handling for invalid proofs.
chain/manager.go	Modifies updateV2TransactionProofs to catch panics and return a descriptive error instead.
.changeset/fixed_a_panic_when_attempting_to_update_element_proofs.md	Documents the change in error handling for element proof updates.

[n8maninger]

@lukechampine @ChrisSchinnerl @peterjan merging this to put out a hot fix, but please still review

[peterjan]

We're not logging panicErr at all in the default case, would it no be better to log failed at %q with panic %#v: %w to ensure we're not needlessly dropping any debug context?