feat(dfget): skip existing output files by default when overwrite is disabled

[NelZyhh]

Background / Motivation

When running dfget repeatedly in batch jobs, retry workflows, or model synchronization tasks,
users often expect idempotent behavior when the output file already exists.

Previously, dfget had only two behaviors for existing output files:

• overwrite the file when --overwrite is enabled
• return an error when --overwrite is not enabled

Based on review feedback, there is no need to introduce a new CLI option for this case.
Instead, when --overwrite=false, dfget should skip existing files by default and return success.

What’s in this PR

Update dfget existing-file behavior without adding a new option:

• --overwrite=true: overwrite the existing output file
• --overwrite=false (default): skip the download and return success if the output file already exists

Additional changes:

• remove the previously introduced --skip-if-exists option
• update argument help text to reflect the new default behavior
• apply the skip behavior before starting a single-file download
• make recursive downloads skip existing files file-by-file through the same logic
• update unit tests accordingly

Usage

Skip existing files by default:

dfget modelscope://Qwen/Qwen3-8B -O ./Qwen3-8B -r

Overwrite existing files explicitly:

dfget modelscope://Qwen/Qwen3-8B -O ./Qwen3-8B -r --overwrite

Compatibility

• this change modifies the default behavior when the output file already exists
• previously, --overwrite=false returned an error
• now, --overwrite=false skips the existing file and returns success
• --overwrite=true behavior remains unchanged

Testing

Local validation:

cargo fmt --all -- --check
cargo clippy --all --all-targets -- -D warnings
cargo test -p dragonfly-client --bin dfget

Runtime verification:

• download to an existing output path without --overwrite
• confirm the command exits successfully and does not overwrite the file
• download to the same path with --overwrite
• confirm the existing file is overwritten

[codecov]

Codecov Report

❌ Patch coverage is 83.33333% with 7 lines in your changes missing coverage. Please review.
✅ Project coverage is 44.20%. Comparing base (667c327) to head (9c90d87).
⚠️ Report is 7 commits behind head on main.

Files with missing lines	Patch %	Lines
dragonfly-client/src/bin/dfget/main.rs	83.33%	7 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1846   +/-   ##
=======================================
  Coverage   44.20%   44.20%           
=======================================
  Files          93       93           
  Lines       27229    27256   +27     
=======================================
+ Hits        12036    12049   +13     
- Misses      15193    15207   +14     

Files with missing lines	Coverage Δ
dragonfly-client/src/bin/dfget/main.rs	50.10% <83.33%> (+0.48%)	⬆️

... and 1 file with indirect coverage changes

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[gaius-qi]

No need to add an option parameter. We can just make "skip if exists" the default behavior.