feat: add DeleteWithConfig/DeleteConfig with generation support to kvstore

[mvanhorn]

Summary

KV Store gets a DeleteWithConfig(key, *DeleteConfig) operation so an if-generation-match precondition can be supplied on deletes, mirroring the IfGenerationMatch field that already landed on the insert side.

Why this matters

#255 (from @kpfleming) asked for two additions; @dgryski noted the InsertConfig half shipped in PR #246, leaving the delete half. This adds the public DeleteWithConfig API plus the ABI plumbing for the if-generation-match precondition on the delete hostcall. The plain Delete(key) signature is unchanged, so existing callers are unaffected, and the non-guest stub (hostcalls_noguest.go) is updated to match the new ABI so the kvstore package keeps compiling under the default build tags.

Testing

Integration test in integration_tests/kvstore/main_test.go covers a generation-matched delete (succeeds) and a stale-generation delete (precondition rejected). go test ./kvstore compiles under both wasip1 and default build tags.

Fixes #255

[mvanhorn]

The test (1.25, 0.41.1) failure was a wasm trap in TestKVStoreDeleteWithConfig/IfGenerationMatch. Root cause: the Viceroy bundled by the CI Fastly CLI (v0.18.0) only defines the reserved delete-config flag in its WITX and its kv_store delete impl ignores the config, while this SDK sets the if_generation_match bit (1 << 1). So the guest traps calling a host that doesn't implement generation-aware delete yet.

No released Viceroy implements it, so rather than bump Viceroy I gated the two generation-aware delete subtests (IfGenerationMatch, StaleGeneration) with t.Skip and that rationale. The SDK code is unchanged and the InsertWithConfig generation tests still run. Happy to switch to a host-version gate or revisit once Viceroy ships delete-with-generation, whichever you prefer.

[dgryski]

Let's put this on hold until we have updated wit file definitions with the appropriate flag values, and the hostcalls have been implemented in viceory. Thanks for all your work!