feat: add methods to flatten credential entry and set secret value

[rbstp]

Change	File	Breaking?	Impact
Form encoding	authentication.go	No	Uses url.Values{} instead of fmt.Sprintf. Same output, but now properly escapes special characters in credentials. Fixes a bug with complex password since they are randomly generated.
RequestError fields	dvls.go	No	Added StatusCode and Body fields. Existing code using Url/Err continues to work.
Error message format	dvls.go	Minor	Error messages now include status code: "...url (status 404)...". Only affects code parsing error strings.
IsNotFound()	dvls.go	No	New function. Additive only.
ToCredentialMap()	entry_credential.go	No	New method on Entry. Additive only.
SetCredentialSecret()	entry_credential.go	No	New method on Entry. Additive only.
Time parsing	server.go	No	Now accepts MORE time formats (RFC3339Nano, etc.). Original format still supported. Fixes parsing failures.

[github-actions]

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[Copilot]

Pull request overview

This PR adds new credential management methods and fixes several bugs in the DVLS Go SDK, including proper URL encoding for authentication credentials, enhanced error handling with status codes, and improved time parsing flexibility.

• Adds ToCredentialMap() and SetCredentialSecret() methods to Entry for credential manipulation
• Enhances RequestError with StatusCode and Body fields and adds IsNotFound() helper function
• Fixes authentication to properly escape special characters in credentials using url.Values
• Improves time parsing to support multiple RFC3339 formats

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 2 comments.

File	Description
authentication.go	Switches from string formatting to url.Values for proper credential encoding, fixing bugs with special characters in passwords
dvls.go	Adds StatusCode and Body fields to RequestError, implements IsNotFound() helper, improves error messages with status codes, and fixes defer placement for response body closure
entry_credential.go	Adds ToCredentialMap() to flatten credential entries into maps and SetCredentialSecret() to update secret values
server.go	Replaces single time format with multiple fallback formats (RFC3339Nano, etc.) for more robust time parsing

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