fix(build-channel): add retry logic and fail on network errors

[JoshuaBatty]

The build-channel script was silently skipping targets when network requests failed, resulting in incomplete channel TOML files with missing platform targets. This caused PRs #812, #813, #814 to have broken channel configurations with randomly missing targets.

Root Cause

The write_document function was handling network errors like this:

Err(e) => {
    eprintln!("Error adding url and hash for target '{}':\n{}", target, e);
}

This silently continued without the target, producing incomplete TOML files.

Changes

• Add fetch_with_retry() helper with exponential backoff (3 retries, 1s/2s/4s delays)
• Replace silent error handling with proper error propagation using ?
• Build now fails if any target cannot be fetched after retries
• Fix clippy warnings for manual string prefix stripping
• Add forc-crypto version to PR description in update-channel workflow (was missing, only 4/5 packages were shown)

[cursor]

PR Summary

Adds retrying downloads with strict error propagation in build-channel and includes forc-crypto version in the update-channel PR description.

• build-channel (Rust):
  • Add fetch_with_retry with exponential backoff and use it for tarball downloads; propagate errors to fail the build instead of skipping targets.
  • Simplify version/tag parsing with strip_prefix for cleaner prefix handling.
• CI workflow (.github/workflows/update-channel.yml):
  • Extract and include forc-crypto version in PR description/env alongside other packages.

^{Written by Cursor Bugbot for commit a7f2898. This will update automatically on new commits. Configure here.}

[github-actions]

LCOV of commit a7f2898 during CI #2292

Summary coverage rate:
  lines......: 87.2% (2671 of 3064 lines)
  functions..: 61.7% (407 of 660 functions)
  branches...: 65.2% (283 of 434 branches)

Files changed coverage rate: n/a