fix: Allow modifying fdv2 data source options independent of main config

[keelerm84]

Note

Medium Risk
Touches core FDv2 synchronization and HTTP wiring; misconfigured defaults or builder overrides could change network behavior (URIs/timeouts) or break initialization if .build() paths are missed.

Overview
Refactors FDv2 DataSystemConfig to use a new DataSourceBuilder protocol (objects with build(config)) instead of Callable[[Config], T] factories, and updates FDv2 (fdv2.py) to call .build() for initializers/synchronizers.

Updates polling and streaming FDv2 data sources to accept per-source overrides (base URI, poll interval / reconnect delay, and HTTPConfig) via new builder classes; requesters now build their own HTTPFactory from _base_headers(config) and the builder-supplied HTTPConfig/URI rather than always using config.http/config.base_uri.

Adds builder implementations for FileDataV2 and TestDataV2, updates integrations/tests/contract tests to use the new builder API, and adjusts StreamingDataSource/SSE client builder signatures accordingly.

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Cursor Bugbot has reviewed your changes and found 1 potential issue.

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Test helper incorrectly appends endpoint to base URI

Low Severity

The test helper make_streaming_data_source() incorrectly passes config.stream_base_uri + STREAMING_ENDPOINT to StreamingDataSource, but the constructor expects just the base URI without the endpoint. The create_sse_client function adds STREAMING_ENDPOINT to the URI it receives, so this would result in a doubled endpoint (e.g., /sdk/stream/sdk/stream). Tests pass because they mock _sse_client_builder, but the helper function is constructing the object incorrectly.