[PM- 22675] Send password auth method (#6228)

* feat: add Passwordvalidation
* fix: update strings to constants
* fix: add customResponse for rust consumption
* test: add tests for SendPasswordValidator. fix: update tests for SendAccessGrantValidator
* feat: update send access constants.

src/Identity/IdentityServer/RequestValidators/SendAccess/SendAccessConstants.cs

@@ -0,0 +1,73 @@
+using Duende.IdentityServer.Validation;
+
+namespace Bit.Identity.IdentityServer.RequestValidators.SendAccess;
+
+/// <summary>
+/// String constants for the Send Access user feature
+/// </summary>
+public static class SendAccessConstants
+{
+    /// <summary>
+    /// A catch all error type for send access related errors. Used mainly in the <see cref="GrantValidationResult.CustomResponse"/>
+    /// </summary>
+    public const string SendAccessError = "send_access_error_type";
+    public static class TokenRequest
+    {
+        /// <summary>
+        /// used to fetch Send from database.
+        /// </summary>
+        public const string SendId = "send_id";
+        /// <summary>
+        /// used to validate Send protected passwords
+        /// </summary>
+        public const string ClientB64HashedPassword = "password_hash_b64";
+        /// <summary>
+        /// email used to see if email is associated with the Send
+        /// </summary>
+        public const string Email = "email";
+        /// <summary>
+        /// Otp code sent to email associated with the Send
+        /// </summary>
+        public const string Otp = "otp";
+    }
+
+    public static class GrantValidatorResults
+    {
+        /// <summary>
+        /// The sendId is valid and the request is well formed.
+        /// </summary>
+        public const string ValidSendGuid = "valid_send_guid";
+        /// <summary>
+        /// The sendId is missing from the request.
+        /// </summary>
+        public const string MissingSendId = "send_id_required";
+        /// <summary>
+        /// The sendId is invalid, does not match a known send.
+        /// </summary>
+        public const string InvalidSendId = "send_id_invalid";
+    }
+
+    public static class PasswordValidatorResults
+    {
+        /// <summary>
+        /// The passwordHashB64 does not match the send's password hash.
+        /// </summary>
+        public const string RequestPasswordDoesNotMatch = "password_hash_b64_invalid";
+        /// <summary>
+        /// The passwordHashB64 is missing from the request.
+        /// </summary>
+        public const string RequestPasswordIsRequired = "password_hash_b64_required";
+    }
+
+    public static class EmailOtpValidatorResults
+    {
+        /// <summary>
+        /// Represents the error code indicating that an email address is required.
+        /// </summary>
+        public const string EmailRequired = "email_required";
+        /// <summary>
+        /// Represents the status indicating that both email and OTP are required, and the OTP has been sent.
+        /// </summary>
+        public const string EmailOtpSent = "email_and_otp_required_otp_sent";
+    }
+}