FCE-2044 validate credentials at client creation

[czerwiukk]

Description

Adds credentials validation to the preferred way of creating the client.
There is also a separate checkCredentials method if someone needs to create the client synchronously.

Motivation and Context

The SDK client should fail fast when misconfigured.

Documentation impact

• Documentation update required
• Documentation updated in another PR
• No documentation update required

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to
  not work as expected)

[linear]

FCE-2044

[Copilot]

Pull request overview

Adds fail-fast configuration/credential validation paths for FishjamClient, introducing an async factory to verify credentials against the backend and a reusable config-validation helper, with accompanying test coverage.

Changes:

• Added validateFishjamConfig and MissingManagementTokenException to validate required config fields at construction time.
• Introduced FishjamClient.create() (async factory) and checkCredentials() to validate credentials via a backend call.
• Added new Vitest coverage for sync config validation and mocked credential-check behavior.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 3 comments.

File	Description
packages/js-server-sdk/tests/config-validation.test.ts	Adds tests for constructor config validation and mocked backend credential checks via getAllRooms.
packages/js-server-sdk/src/utils.ts	Introduces validateFishjamConfig to enforce presence of fishjamId and managementToken.
packages/js-server-sdk/src/exceptions/index.ts	Adds MissingManagementTokenException for missing/empty management token config.
packages/js-server-sdk/src/client.ts	Validates config in constructor; adds create() async factory and checkCredentials() method with updated JSDoc.

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