Allow pants export --resolve to take targets

[wisechengyi]

Summary

• Removes the guard that rejected target specs in the export goal
• When targets are provided with --resolve, the exported virtualenv contains only the 3rd-party requirements transitively needed by those targets (via Pex native lockfile subsetting)
• When no targets are provided, the existing full-resolve behavior is unchanged

Example usage:

pants export --resolve=python-default src/python/my_service:my_service

Test plan

• Existing test_export_venv_new_codepath, test_export_tool, and test_export_codegen_outputs still pass (full-resolve path unchanged)
• New test_export_venv_filtered_by_targets verifies targets flow through the rule chain and the transitive-closure branch is taken

Adhoc tests

Full resolve (no targets) — baseline unchanged:

$ pants export --resolve=python-default
Wrote mutable virtualenv for python-default (using Python 3.14.3) to dist/export/python/virtualenvs/python-default/3.14.3
# 30+ packages installed

Filtered by a source target with no 3rd-party deps (src/python/pants/core/goals/export.py):

$ pants export --resolve=python-default src/python/pants/core/goals/export.py
Wrote mutable virtualenv for python-default (using Python 3.14.3) to dist/export/python/virtualenvs/python-default/3.14.3
# site-packages: colors  packaging  pip  typing_extensions  (PEX builtins only — no extra 3rd-party deps)

Filtered by a test file with pytest in its transitive closure (src/python/pants/backend/python/goals/export_test.py):

$ pants dependencies --transitive src/python/pants/backend/python/goals/export_test.py | grep 3rdparty
3rdparty/python#ansicolors
3rdparty/python#hdrhistogram
3rdparty/python#packaging
3rdparty/python#pytest
3rdparty/python#toml
3rdparty/python#types-toml
3rdparty/python#typing-extensions

$ pants export --resolve=python-default src/python/pants/backend/python/goals/export_test.py
Wrote mutable virtualenv for python-default (using Python 3.14.3) to dist/export/python/virtualenvs/python-default/3.14.3
# site-packages: hdrh  iniconfig  pluggy  pygments  pytest  _pytest  setuptools  toml  toml-stubs  ...

pytest and its transitive deps are present; unrelated packages from the full resolve are absent.

🤖 Generated with Claude Code

[wisechengyi]

Potentially worth reviewing whether this is an overkill. E.g. do we need to care at export phase? For targets that can't build due to diverging resolves, is it okay to ignore the failure during export?

[cburroughs]

Thanks for the contribution. We've just branched for 2.33.x, so merging this pull request now will come out in 2.34.x, please move the release notes updates to docs/notes/2.34.x.md if that's appropriate.