fix: reject kyc remove vc through did

[x0tta6bl4-ai]

Fixes #365.

Summary

• reject generic DID RemoveVC requests for the reserved KYC service id
• add a DID keeper regression test proving KYC credentials and filters remain intact when the generic DID route is used
• add a dedicated DID error for reserved credential services

Tests

• go test ./x/did/keeper -run TestMsgServerRemoveVCRejectsKycReservedSid -count=1 -v
• go test ./x/did/keeper -count=1 -v
• go test ./x/did/types -count=1
• git diff --check
• git diff --cached --check

Baseline notes reproduced on clean origin/main at 9ca8f8fb:

• go test ./x/did -count=1 -v fails in existing TestInitExportGenesis with empty address string is not allowed
• go test ./x/kyc/types -count=1 fails in existing TestGenesisState_Validate expectations

Bounty payout wallet: 0x6017613e80d7893EB2aD5c0585b3f1f88CD6e099.