Make the payment secret field ('s') mandatory for BOLT11 payment requests,
implementing the requirement specified in BOLT11 spec PR #1242
(https://github.com/lightning/bolts/pull/1242).
This security enhancement prevents payment probing attacks by requiring
all invoices to include payment secrets. Changes include:
1. Adding validation in bolt11_decode_nosig() to reject invoices without
the 's' field
2. Adding payment secrets to all test vectors
3. Updating expected encoded values in test cases to include payment secrets
4. Adding a specific test case that verifies proper rejection of invoices
missing the payment secret field
Changelog-Changed: Made payment secret ('s' field) mandatory in BOLT11 payment requests for improved security.
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