fetch: add --num option to fetch the last N commits

[miloudbelarebia]

Closes #11033

Problem

When merging long-lived branches, DVC can hit missing cache from earlier
commits (No such file or directory: .../xx/xxx.dir). The current workaround
is dvc fetch --all-commits, which on a branch like main can pull a huge
amount of data that isn't actually needed.

Solution

Add a --num <N> option to dvc fetch, mirroring the existing dvc gc --num:

dvc fetch --num 10

• Used on its own, it fetches the cache for the last N commits walking back
  from the current branch's HEAD.
• Combined with --all-branches, it applies to each branch tip (consistent
  with how num already behaves in dvc gc / brancher).
• --num defaults to 1 (current behavior, workspace only). Values < 1
  raise InvalidArgumentError. A value larger than the available history is
  capped at the number of existing commits.

The plumbing reuses the num parameter already threaded through
Repo.brancher / iter_revs, so the change is small: thread num through
fetch() and _collect_indexes(), and expose the CLI flag.

Tests

• tests/func/test_data_cloud.py::test_fetch_num — fetches the right number
  of commit versions for num=1/2/10.
• tests/func/test_data_cloud.py::test_fetch_num_invalid — num=0 raises.
• Updated tests/unit/command/test_data_sync.py::test_fetch to cover the flag.

All pass locally; ruff check / ruff format --check are clean.

[CLAassistant]

All committers have signed the CLA.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.98%. Comparing base (2431ec6) to head (3ab3f6a).
⚠️ Report is 204 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #11047      +/-   ##
==========================================
+ Coverage   90.68%   90.98%   +0.30%     
==========================================
  Files         504      505       +1     
  Lines       39795    41159    +1364     
  Branches     3141     3265     +124     
==========================================
+ Hits        36087    37449    +1362     
- Misses       3042     3071      +29     
+ Partials      666      639      -27     

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