feat!: Add BRAINTRUST_CAPTURE_ATTACHMENTS env var option and captureAttachments logger option for all integrations

[Luca Forstner (lforst)]

Ref braintrustdata/braintrust-sdk-javascript#1841

• Make automatic attachment capture opt-in across all integrations using the existing BRAINTRUST_CAPTURE_ATTACHMENTS environment variable; previously, integrations applied this setting inconsistently.
• Add initLogger({ captureAttachments: true | false }) to override the environment setting. Loggers created with setCurrent: false keep their own policy, independent of the global logger.
• When capture is disabled, omit inline media and skip unnecessary decoding, copying, and buffering. Preserve text, metadata, metrics, remote URLs, and explicitly logged attachments.
• Keep the owning logger’s policy throughout asynchronous calls and streaming responses, with unit, integration, and replay tests covering these behaviors.