chore: Extract shared sans-I/O cores and add config read-protocols

[jsonbailey]

What

Two pieces of async-SDK foundation, laid on the sync side and behavior-identical to main:

1. Shared sans-I/O _common modules. Pulls I/O-free logic out of the sync data paths into impl/client_common.py, impl/datasystem/fdv2_common.py, impl/events/event_processor_common.py — imported by both the sync code and the future async siblings. client.py / event_processor.py / fdv2.py route through them; fdv2.py re-exports the moved names via __all__ so existing import paths still resolve.

2. Config read-protocols + build() widening. Introduces structural Protocols in config.py describing the config surface each consumer reads, so a future AsyncConfig can satisfy the same contracts without a fork:

• SdkIdentityConfig — sdk_key, wrapper_name, wrapper_version, application; parameter type of get_environment_metadata / secure_mode_hash.
• DataSourceBuilderConfig(SdkIdentityConfig) — adds base_uri, stream_base_uri, http, initial_reconnect_delay, poll_interval, payload_filter_key, _instance_id; parameter type of DataSourceBuilder.build().
• PrivateAttributesConfig — all_attributes_private, private_attributes; parameter type of EventOutputFormatter.

Config explicitly inherits DataSourceBuilderConfig + PrivateAttributesConfig. DataSourceBuilder.build() and the datasourcev2 data sources are widened from Config to DataSourceBuilderConfig. Fully backward-compatible: Config satisfies all three protocols, so every existing caller works unchanged.

Why

Foundation for the async Python SDK (epic SDK-60). The _common modules + config protocols let the sync and async clients share identical I/O-free behavior and config contracts instead of duplicating them. The async siblings and AsyncConfig land in separate slices.

Parity

Behavior-identical to main: the _common extraction moves code verbatim, and the protocols/build() widening are types-only (Config satisfies the protocols; build() still receives a Config at runtime). Worth a careful sync-behavior review.

Validation

• uv run mypy ldclient — clean
• uv run isort --check ldclient contract-tests / uv run pycodestyle ldclient contract-tests — clean
• uv run pytest ldclient/testing --ignore=integrations — 935 passed

Refs SDK-2602 (epic SDK-60).