feat: rename bulk environment variables

[nabil-Tounarti]

No description provided.

[Copilot]

Pull Request Overview

This PR renames bulk environment variables to use a consistent IEXEC_ prefix and consolidates the dataset variable handling into indexed variants. The changes standardize the naming convention and simplify the code by unifying how single and bulk datasets are accessed.

• Renamed BULK_SIZE to IEXEC_BULK_SLICE_SIZE and consolidated BULK_DATASET_* variables into indexed IEXEC_DATASET_* variants
• Unified dataset handling logic to use a single loop that handles both single (index 0) and bulk datasets (index 1+)
• Updated all references, tests, and documentation to reflect the new naming convention

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
pre-compute/src/compute/utils/env_utils.rs	Updated enum variants and environment variable name mapping to use IEXEC_ prefix and indexed dataset variables
pre-compute/src/compute/pre_compute_args.rs	Consolidated dataset loading logic, renamed field from bulk_size to iexec_bulk_slice_size, updated documentation and tests
pre-compute/src/compute/pre_compute_app.rs	Updated test to use new field name iexec_bulk_slice_size

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