FCE-2044. sdk clients should fail fast

[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 credential validation to the Python Fishjam SDK so misconfigured clients/notifiers error immediately, and provides an explicit “construct + live verify” path for callers that want backend validation up front.

Changes:

• Introduces MissingFishjamIdError / MissingManagementTokenError and a shared validate_fishjam_config() helper.
• Ensures Client (and therefore FishjamClient) and FishjamNotifier validate required credentials during construction.
• Adds FishjamClient.create_and_verify() and FishjamClient.check_credentials() plus new unit tests for config validation / live-check behavior.

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
tests/test_config_validation.py	Adds unit tests covering constructor validation and create_and_verify / check_credentials behaviors.
fishjam/utils.py	Adds validate_fishjam_config() utility used by client/notifier constructors.
fishjam/errors.py	Adds new public exceptions for missing fishjam_id / management_token.
fishjam/api/_fishjam_client.py	Documents constructor validation semantics and adds create_and_verify() + check_credentials().
fishjam/api/_client.py	Enforces credential validation in the shared base client constructor.
fishjam/_ws_notifier.py	Enforces credential validation for the WebSocket notifier constructor.
fishjam/__init__.py	Re-exports the new validation exception types as part of the public package API.

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