fix(transaction): validate endorsements count on merge

[Prachi194agrawal]

Type of change

• Bug fix

Description

This change prevents Merge from panicking when a transaction carries more
endorsement entries than namespace entries.

Previously, mergeEndorsements allocated its internal slices using
len(txs[0].GetNamespaces()), but then indexed them using positions from
tx.GetEndorsements(). When len(Endorsements) > len(Namespaces) for any
transaction, this led to an index out of range panic.

To fix this, validateTransactionsForMerge now checks that, for every
transaction, the endorsements count matches the base namespace count. If not,
Merge returns a descriptive error instead of panicking.

A test case was added to TestMerge_ErrorCases to cover the endorsements
count mismatch scenario.

Additional details (Optional)

The new validation leverages the existing baseNsCount computed from the
first transaction, so the check is performed consistently with the existing
namespace-count validation. The error message includes the transaction index
and both counts to aid debugging.

Tests:

• go test ./tools/fxconfig/internal/transaction -run TestMerge_ErrorCases -v

Related issues

• bug(fxconfig/transaction): Merge panics with index out of range when Endorsements count exceeds Namespaces count #238