fix: make redacted credential fields nullable on response types

[riteshptl21]

Fixes #4.

The API redacts secrets on read and returns null for credential fields, but 93 type files declare them as non-nullable string — so deserializing any response containing a redacted credential throws a TypeError and fails the entire call (a single BYO SIP number breaks the whole phoneNumbers->list() response, for example).

What this changes

All 95 affected field instances across 93 files (apiKey, credentialId, apiSecret, awsSecretAccessKey, password, token, accessToken, sessionToken, secretKey, secretAccessKey, clientSecret, tokenUri), mechanically:

-     * @var string $password ...
+     * @var ?string $password ...
-    public string $password;
+    public ?string $password;
-     *   password: string,
+     *   password?: ?string,
-        $this->password = $values['password'];
+        $this->password = $values['password'] ?? null;

The write path is unchanged: values supplied at the call site serialize exactly as before.

Verification

• The reproduction from Credential fields are typed non-nullable, so any redacted secret makes the whole response undecodable #4 now deserializes with password === null instead of throwing
• composer test: 52 tests, 220 assertions, all passing
• composer analyze (PHPStan level max): no errors
• php -l clean on all 93 changed files

Note for maintainers

Since this repo is Fern-generated, these files will be overwritten on the next regeneration — the durable fix is marking redacted-on-read credential fields as nullable in the API definition/generator config. This PR fixes the published SDK in the meantime and doubles as the exact change list for the generator-side fix.