refactor: give reasoning decisions their own module

providers.py had been holding two jobs: the registry of what each
provider looks like, and the decisions made from those declarations.
Adding response-side judgement would have made it the module for
everything about reasoning, so the decisions move to thinking.py and
the registry keeps only profiles and their lookup.

Moving a module breaks any deep-path import of what moved, so the six
public symbols are promoted to the package root at the same time. The
top level is this library's stated API surface; giving downstream a
stable name to import is what makes the next reorganisation harmless.
observe_thinking stays unexported — downstream reads the verdict off
LLMResponse, and exporting it would be a permanent promise for nothing.

src/polygateway/transports/openai_compat.py

@@ -22,15 +22,14 @@ from polygateway.errors import (
     SourceDeadError,
     TransientError,
 )
-from polygateway.providers import (
-    ProviderProfile,
+from polygateway.providers import ProviderProfile, get_provider
+from polygateway.streaming import StreamLivenessTimeout, stream_with_liveness_timeouts
+from polygateway.thinking import (
     ThinkingCapability,
     ThinkingUnsupportedError,
     get_capability,
-    get_provider,
     resolve_thinking,
 )
-from polygateway.streaming import StreamLivenessTimeout, stream_with_liveness_timeouts
 from polygateway.transports._http_errors import compose_message, summarize_body
 from polygateway.types import EmbeddingTransportResult, SourceConfig, TransportResult