Fix ledger_viz.py and split_ledger.py for COSE-only ledgers

[Copilot]

*UPD. I did a lot than this locally, but the idea holds

• Fix python/src/ccf/ledger_viz.py to detect signatures in public:ccf.internal.cose_signatures
• Fix python/src/ccf/split_ledger.py to recognise COSE-only signatures (both for splitting at a sig seqno and looking for the next signature)
• Make split_all_ledger_files_in_dir (test infra) recognise COSE-only signature tables
• Add an offline run_ledger_viz_test that calls ccf.ledger_viz.visualise() in-process on a checked-in Dual->COSE upgraded ledger and matches against a golden output file
• Add an offline run_split_ledger_test that runs split_ledger on the same checked-in ledger and verifies the result still reads cleanly
• Update CHANGELOG.md

[Copilot]

Pull request overview

This PR updates the Python ledger tooling and test infrastructure to correctly detect signature transactions in COSE-only ledgers (where signatures are recorded in public:ccf.internal.cose_signatures), and adds offline regression tests using checked-in upgraded-ledger testdata.

Changes:

• Teach ledger_viz and split_ledger to treat transactions containing COSE_SIGNATURE_TX_TABLE_NAME as signature transactions.
• Update test infra ledger-splitting logic and add offline tests to validate ledger_viz output and split_ledger behavior on a Dual→COSE upgraded ledger.
• Add a CHANGELOG entry for the fix.

Custom instructions used:

• .github/copilot-instructions.md
• .github/instructions/reviewing.instructions.md
• .github/instructions/changelog.instructions.md

Reviewed changes

Copilot reviewed 5 out of 25 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
python/src/ccf/ledger_viz.py	Refactors iteration into visualise() and classifies COSE signature txs as “Signature”.
python/src/ccf/split_ledger.py	Recognises COSE signature txs both when splitting at a sig seqno and when searching for the next signature.
tests/e2e_operations.py	Updates signature detection for splitting and adds offline regression tests for ledger_viz + split_ledger.
tests/testdata/cose_upgraded_service/expected_viz.txt	Adds golden output for the offline ledger_viz regression test.
tests/testdata/cose_upgraded_service/snapshots/.gitkeep	Keeps the snapshots directory present in the checked-in testdata.
CHANGELOG.md	Documents the fix (but currently introduces a duplicate “Fixed” heading under 7.0.3).