fix(typescript): resolve type errors in gasless deposit tests

Add type assertions to test cases that intentionally use invalid
chain names for error handling validation. This fixes TypeScript
compilation errors while preserving the runtime validation tests.

Changes:
- test/services/deposits.test.ts: Add 'as GaslessDestination' type
  assertions to 5 test cases that verify rejection of invalid chain
  names (Optimism, arbitrum, InvalidChain, empty string)
- api-reference/classes/DepositsService.md: Update parameter type
  documentation from 'string' to 'GaslessDestination'

The type assertions allow TypeScript to compile while the tests
still properly validate runtime error handling for unsupported
chain names.

Author: lrsaturnino

typescript/api-reference/classes/DepositsService.md

@@ -466,7 +466,7 @@ proper extraData encoding for the destination chain.
 | :------ | :------ | :------ |
 | `bitcoinRecoveryAddress` | `string` | P2PKH or P2WPKH Bitcoin address for emergency recovery |
 | `depositOwner` | `string` | Ethereum address that will receive the minted tBTC. For L1 deposits, this is the user's Ethereum address. For L2 deposits, this is typically the signer's address (obtained from the destination chain BitcoinDepositor). |
-| `destinationChainName` | `string` | Target chain name for the deposit. Must be one of the supported chains (case-sensitive): - "L1" - Direct L1 deposits via NativeBTCDepositor - "Arbitrum" - Arbitrum L2 deposits - "Base" - Base L2 deposits - "Sui" - Sui L2 deposits - "StarkNet" - StarkNet L2 deposits (note: capital 'N') Note: "Solana" is not currently supported for gasless deposits |
+| `destinationChainName` | [`GaslessDestination`](../README.md#gaslessdestination) | Target chain name for the deposit. Must be one of the supported chains (case-sensitive): - "L1" - Direct L1 deposits via NativeBTCDepositor - "Arbitrum" - Arbitrum L2 deposits - "Base" - Base L2 deposits - "Sui" - Sui L2 deposits - "StarkNet" - StarkNet L2 deposits (note: capital 'N') Note: "Solana" is not currently supported for gasless deposits |
 
 #### Returns
 

typescript/test/services/deposits.test.ts

@@ -3263,7 +3263,7 @@ describe("Deposits", () => {
             depositService.initiateGaslessDeposit(
               "mjc2zGWypwpNyDi4ZxGbBNnUA84bfgiwYc",
               "0x742d35Cc6634C0532925a3b844Bc9e7595f0bEb1",
-              "Optimism"
+              "Optimism" as GaslessDestination
             )
           ).to.be.rejectedWith(/Gasless deposits are not supported for chain/)
         })
@@ -3273,7 +3273,7 @@ describe("Deposits", () => {
             depositService.initiateGaslessDeposit(
               "mjc2zGWypwpNyDi4ZxGbBNnUA84bfgiwYc",
               "0x742d35Cc6634C0532925a3b844Bc9e7595f0bEb1",
-              "arbitrum"
+              "arbitrum" as GaslessDestination
             )
           ).to.be.rejectedWith(/Gasless deposits are not supported for chain/)
         })
@@ -3283,7 +3283,7 @@ describe("Deposits", () => {
             await depositService.initiateGaslessDeposit(
               "mjc2zGWypwpNyDi4ZxGbBNnUA84bfgiwYc",
               "0x742d35Cc6634C0532925a3b844Bc9e7595f0bEb1",
-              "InvalidChain"
+              "InvalidChain" as GaslessDestination
             )
             expect.fail("Should have thrown an error")
           } catch (error: any) {
@@ -3748,7 +3748,7 @@ describe("Deposits", () => {
             depositService.initiateGaslessDeposit(
               "mjc2zGWypwpNyDi4ZxGbBNnUA84bfgiwYc",
               "0x742d35Cc6634C0532925a3b844Bc9e7595f0bEb1",
-              "InvalidChain"
+              "InvalidChain" as GaslessDestination
             )
           ).to.be.rejectedWith(/Gasless deposits are not supported for chain/)
         })
@@ -3758,7 +3758,7 @@ describe("Deposits", () => {
             depositService.initiateGaslessDeposit(
               "mjc2zGWypwpNyDi4ZxGbBNnUA84bfgiwYc",
               "0x742d35Cc6634C0532925a3b844Bc9e7595f0bEb1",
-              ""
+              "" as GaslessDestination
             )
           ).to.be.rejectedWith(/Gasless deposits are not supported for chain/)
         })