feat: Allow deferred chat client initialization

[cpsievert]

Summary

QueryChat now supports deferred client creation in both Python and R. The chat client (chatlas Chat / ellmer Chat) no longer needs to exist at construction time — you can pass it later via server(client=...), scoped to the current Shiny session.

This unlocks Posit Connect managed OAuth workflows where credentials are only available inside the Shiny server function.

# Python — no API key needed at construction
qc = QueryChat(None, "mtcars", greeting=greeting)

def server(input, output, session):
    qc.server(client=ChatAnthropic(), data_source=mtcars)

# R — same pattern
qc <- QueryChat$new(NULL, "mtcars", greeting = greeting)

server <- function(input, output, session) {
  qc$server(client = chat_anthropic(), data_source = mtcars)
}

Under the hood

• Store the client "spec" (str | Chat | NULL) instead of eagerly resolving it at init time
• Add _create_session_client() / create_session_client() as the single resolution point that turns a spec into a fully-configured Chat with system prompt and tools
• server(client=...) captures the override in a session-local closure so it never mutates the shared spec — safe for concurrent sessions
• Clone the resolved Chat on every call to prevent mutation of a shared Chat object

Closes #205

Test plan

• Deferred init: QueryChat(NULL, ...) succeeds without API key
• Guard rails: $client(), $console(), $generate_greeting() error when data_source is still NULL
• Deferred then resolved: set data_source later, everything works
• Session isolation: server(client=...) override doesn't leak to other sessions
• Clone safety: multiple $client() calls with a Chat spec return independent instances
• Backward compat: existing patterns with immediate data_source continue to work
• Full test suites pass (Python and R)

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