Remove interrupt from StoredFlow, function stores and public API

[stidsborg]

Removes the interrupt mechanism entirely, now unused after the push-only message delivery migration (#180).

Changes

• Remove Interrupted from StoredFlow and both Interrupt overloads from IFunctionStore (in-memory, PostgreSQL, SqlServer and MariaDB implementations)
• Remove the public BaseRegistration.Interrupt API; BaseRegistration no longer depends on IFunctionStore
• Drop the interrupted column and its index from all three store schemas, and remove it from all insert/select/update statements (reader ordinals shifted accordingly)
• Simplify state transitions now that the flag is gone: suspend always suspends and postpone always uses the given expiry (previously CASE WHEN interrupted ...)
• Remove leftover interrupt-merge in SqlServer SqlGenerator.AppendMessages, aligning it with the PostgreSQL and MariaDB append behavior
• Remove interrupt-based tests and update the stale IMessageStore.AppendMessages doc comment

Verification

All test suites pass against reset databases: core (511), MariaDB (368), PostgreSQL (368), SqlServer (368).

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