- Refactor constant definitions and organization
- Clean up package level variables and functions
- Introduce new `relaymode` and `apitype` packages for constant definitions
- Refactor and simplify code in several packages including `openai`, `relay/channel/baidu`, `relay/util`, `relay/controller`, `relay/channeltype`
- Add helper functions in `relay/channeltype` package to convert channel type constants to corresponding API type constants
- Remove deprecated functions such as `ResponseText2Usage` from `relay/channel/openai/helper.go`
- Modify code in `relay/util/validation.go` and related files to use new `validator.ValidateTextRequest` function
- Rename `util` package to `relaymode` and update related imports in several packages
- Added required packages to go.mod: `github.com/Laisky/errors/v2 v2.0.1`, `github.com/stretchr/testify v1.8.3`, `github.com/davecgh/go-spew v1.1.1`, `github.com/pmezard/go-difflib v1.0.0`
- Increased the number of returned recordings to 100 in `relay-utils.go`
- Refactored and simplified code in `relay-aiproxy.go` for improved message retrieval and error handling
- Added new test file `relay_test.go` and various test cases for different message types
- Modified functions in `group.go` and `relay.go` for improved functionality
