Support non-standard expires_in responses in OAuthTokensSchema

[linucs]

Describe the bug
During the OAuth token exchange, some providers (e.g. NetSuite) do not respond following the RFC 6749 specs, and return the expires_in field as a numeric string.
It would be helpful to support such uses cases; to solve the issue, I simply applied this patch to https://github.com/modelcontextprotocol/typescript-sdk/blob/main/src/shared/auth.ts (line 139):

expires_in: z.coerce.number().optional(),

To Reproduce
Steps to reproduce the behavior:

1. Setup an integration record in NetSuite
2. Create a client and start the OAuth process
3. See the authentication succeeding
4. After the code is received, see the token request-response failing with the following message:

Error: Invalid token response from OAuth provider
Expected 'expires_in' to be a number, received string

Expected behavior
Token expiration is parsed as a numeric string.

Logs
See the above error message.

Additional context
Tested against NetSuite 2025.2.