fix(weighted-validator): handle initial UserOp signatures

[anshitraj]

Summary

• handle an undefined signature while preparing the first UserOperation
• replace this signer's dummy stub before appending the real signature
• preserve signatures supplied by other weighted signers
• remove the debug log from stub generation
• add regression coverage and a patch changeset

Closes #264.

Testing

• npx bun test plugins/weighted-r1-k1/toWeightedValidatorPlugin.test.ts
• npx bun x biome check plugins/weighted-r1-k1/toWeightedValidatorPlugin.ts plugins/weighted-r1-k1/toWeightedValidatorPlugin.test.ts
• bundled the updated validator entry with Bun