refactor(ts-client): replace string literal errors with SdkErrorCode enum

[Shawnchee]

Summary

Replaces the ErrorName string literal union with an SdkErrorCode enum
for SDK-side errors, centralises default error messages, and adds a
type-safe DlmmSdkError.is() helper for catch blocks

Problem

• Error names are raw strings and every throw site must hardcode a message
• No type-safe way to check error types in catch blocks

Changes

ts-client/src/dlmm/error.ts

• Added SdkErrorCode enum with existing error names
• Added SDK_ERROR_MESSAGES map with default messages
• Updated DlmmSdkError constructor: errorMessage is now optional,
  falls back to SDK_ERROR_MESSAGES
• Added DlmmSdkError.is() static type predicate

ts-client/src/dlmm/index.ts

• Updated 5 throw sites to use SdkErrorCode enum
• Removed unused imports: getSimulationComputeUnits,
  ComputeBudgetInstruction

ts-client/src/test/sdk_error.test.ts (newly added)

• 8 unit tests covering constructor, default/custom messages,
  and static is() method

Breaking Changes

None. Enum values match the original string literals. Existing
error handling code continues to work

Testing

cd ts-client
npx jest src/test/sdk_error.test.ts