feat(py): Allow deferred chat client initialization

[cpsievert]

Summary

• Allow client to be deferred when data_source=None and no explicit client is provided
• Add chat_client property getter/setter for setting client after initialization
• Add client parameter to server() method for deferred pattern
• Add _require_client() method for runtime checks on methods that need the client

Motivation

When using Posit Connect managed OAuth credentials, all data/model connections must be created in the Shiny server function, as connections using managed credentials need access to the HTTP headers in the Shiny session object. Previously, when creating the QueryChat class without an explicit client, it would attempt to connect to OpenAI, resulting in an exception when OPENAI_API_KEY wasn't set.

Now, when data_source=None (the deferred pattern), the client is also deferred by default:

# No error - client is deferred when data_source is None
qc = querychat.QueryChat(
    None,
    table_name="mtcars",
    greeting=greeting,
    data_description=data_desc
)

def server(input, output, session):
    # Set client in server where session credentials are available
    qc_vals = qc.server(
        client=ChatAnthropic(),
        data_source=mtcars_table
    )

Closes #205

Test plan

• Added unit tests for deferred client initialization pattern
• Added tests for chat_client property setter
• Added tests that methods properly require client to be set
• Added integration tests for full deferred workflow
• Updated existing deferred data source test
• All 301 tests pass

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