ONLY COSE receipts in snapshots

[maxtropets]

Towards #7401.

Snapshots now carry COSE-signed receipts, which will be supported both by old ledgers signing both ways (classic + COSE), and new (COSE-only).

This will also enable PQC via COSE signature in the future, as the receipt format will remain COSE.

This needs #7712 to be merged for backwards compatibility with 6.x.

[Copilot]

Pull request overview

Updates snapshot receipt handling to use COSE receipts (while retaining support for parsing legacy JSON receipts), enabling COSE-only ledgers and future PQC-friendly receipts.

Changes:

• Switch snapshot generation to record/store COSE signatures rather than classic (node-cert-based) signatures.
• Extend snapshot receipt deserialisation/verification to detect and handle COSE vs JSON receipt formats.
• Update Python snapshot parsing to verify COSE receipts and update recovery test expectations.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
tests/recovery.py	Updates expected error string when snapshot signer doesn’t match the provided previous service identity.
src/node/test/snapshotter.cpp	Adjusts snapshotter test helper to record a dummy COSE signature instead of classic signature fields.
src/node/snapshotter.h	Removes classic signature fields from pending snapshot info and records COSE signatures for snapshot receipts.
src/node/snapshot_serdes.h	Adds COSE receipt parsing/verification path and switches snapshot receipt serialisation to COSE.
src/node/node_state.h	Hooks snapshotter signature recording to the COSE signatures table rather than classic signatures.
python/src/ccf/ledger.py	Adds snapshot receipt verification dispatch (COSE vs JSON) and COSE snapshot receipt verification using service identity from the snapshot.