feat(dfctl): add task preheat command for image and file preheat

[Copilot]

Adds dfctl task preheat to trigger preheat of OCI images (oci:// prefix) or files (http:///https:// prefix) through the Dragonfly scheduler.

Two modes

• gRPC (default): Directly calls scheduler's PreheatImage/PreheatFile RPCs.
• SDK (--request-sdk): Preheats via the Dragonfly SDK proxy — uses Proxy.preheat() for images and Proxy.get() for files from dragonfly-client-util::request.

Changes

• dragonfly-client/src/grpc/scheduler.rs — Added preheat_image and preheat_file methods to SchedulerClient, wrapping the proto RPCs from dragonfly-api.
• dragonfly-client/src/bin/dfctl/task.rs — Added Preheat variant to TaskSubCommand with URL-scheme-based dispatch and full CLI arg set (--scheduler, --request-sdk, --platform, --username, --password, --piece-length, --tag, --application, --timeout).
• dragonfly-client/Cargo.toml — Added oci-client workspace dep for OCI reference parsing (manifest URL construction in gRPC mode).

Usage

# Image preheat via scheduler gRPC
dfctl task preheat oci://docker.io/library/nginx:latest -s http://scheduler:8002

# File preheat via scheduler gRPC
dfctl task preheat https://example.com/file.tar.gz -s http://scheduler:8002

# Image preheat via SDK proxy
dfctl task preheat oci://docker.io/library/nginx:latest --request-sdk -s http://scheduler:8002

# With registry auth and platform selection
dfctl task preheat oci://registry.example.com/app:v1 --username user --password pass --platform linux/amd64

[codecov]

Codecov Report

❌ Patch coverage is 0.77778% with 893 lines in your changes missing coverage. Please review.
✅ Project coverage is 43.11%. Comparing base (bea655e) to head (9b31dc5).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
dragonfly-client/src/bin/dfctl/task.rs	0.00%	635 Missing ⚠️
...nfly-client/src/bin/dfctl/persistent_cache_task.rs	0.00%	125 Missing ⚠️
dragonfly-client/src/bin/dfctl/persistent_task.rs	0.00%	125 Missing ⚠️
dragonfly-client-util/src/request/mod.rs	0.00%	6 Missing ⚠️
dragonfly-client/src/bin/dfget/main.rs	77.77%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1781      +/-   ##
==========================================
- Coverage   44.43%   43.11%   -1.33%     
==========================================
  Files          91       91              
  Lines       26054    26839     +785     
==========================================
- Hits        11578    11571       -7     
- Misses      14476    15268     +792     

Files with missing lines	Coverage Δ
dragonfly-client-backend/src/lib.rs	96.30% <ø> (ø)
dragonfly-client/src/bin/dfget/main.rs	49.33% <77.77%> (ø)
dragonfly-client-util/src/request/mod.rs	32.82% <0.00%> (ø)
...nfly-client/src/bin/dfctl/persistent_cache_task.rs	0.00% <0.00%> (ø)
dragonfly-client/src/bin/dfctl/persistent_task.rs	0.00% <0.00%> (ø)
dragonfly-client/src/bin/dfctl/task.rs	0.00% <0.00%> (ø)

... and 1 file with indirect coverage changes

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[Copilot]

Pull request overview

Adds a new dfctl task preheat subcommand to trigger scheduler-side preheating for OCI images (oci://…) and file/object URLs, with both direct scheduler gRPC mode and an optional Request SDK (--request-sdk) mode.

Changes:

• Added dfctl task preheat with URL-scheme dispatch and extensive CLI flags for scheduler endpoint, auth, platform, piece length, scope, and timeouts.
• Improved dfctl-family CLI error rendering by attempting to decode structured backend error details from gRPC status details.
• Updated dragonfly-client-util::request API to make request/response headers non-optional, and added a tiny OCI scheme constant in the backend crate.

Reviewed changes

Copilot reviewed 4 out of 8 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
dragonfly-client/src/bin/dfctl/task.rs	Adds task preheat command implementation and extends error formatting for task list/remove/preheat paths.
dragonfly-client/src/bin/dfctl/persistent_task.rs	Extends persistent task listing error formatting (backend error details + headers).
dragonfly-client/src/bin/dfctl/persistent_cache_task.rs	Extends persistent cache task listing error formatting (backend error details + headers).
dragonfly-client/Cargo.toml	Adds oci-client dependency for OCI reference parsing in dfctl preheat.
dragonfly-client-util/src/request/mod.rs	Makes GetRequest.header and GetResponse.header non-optional and updates internal handling accordingly.
dragonfly-client-backend/src/oci.rs	Introduces oci URL scheme constant.
dragonfly-client-backend/src/lib.rs	Exposes the new oci module.
Cargo.lock	Locks the new oci-client dependency.

[BraveY]

LGTM

[chlins]

lgtm