ES-2944 - use this (self) instead of bound ls

[jgrantr]

• To make mocking things for unit tests easier, change the functions (that were already function and not arrow functions) to use this instead of ls. This allows us to easily mock the functions that send the data to S3 or various other places without having to re-implement the streams module.
• ensured the unit tests all passed before and after (with sufficient coverage)
• only test modified was one from the mock SDK, none that exercise the real pipelines

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Note

Medium Risk
Touches core stream pipeline assembly (load/enrich/offload and putEvent) and changes method binding semantics, which could break callers that relied on the previous closure-bound ls behavior or unbound function references.

Overview
Refactors leo-stream.js pipeline builders (load, enrich, offload, and putEvent) to use this (captured as self) instead of the closed-over ls, so overridden stream methods are honored during runtime and testing.

Updates the public SDK (index.js) to bind load/enrich/offload to the underlying stream instance, and simplifies lib/mock-sdk.ts by deleting the custom enrich/offload reimplementations and explicitly binding mock enrich/offload so this resolves to mockStreams (plus a small unit test adjustment around disableS3: false).

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