Enable bias by default for linear models

[Same0620]

What does this PR do?

This PR enables bias by default for linear models. When -B is omitted,
the default bias is now 1.0, making the behavior equivalent to passing
-B 1.

The effective bias value is resolved before deciding whether to append
the bias feature column. Positive -B values continue to override the
default, and -B -1 can still be used to disable the bias term.

Test CLI & API (bash tests/autotest.sh)

Test APIs used by main.py.

• Test Pass
  • All tests finished (13/13) on autotest/local-run. See test_report.txt for details.
• Not Applicable (i.e., the PR does not include API changes.)

Check API Document

If any new APIs are added, please check if the description of the APIs is added to API document.

• API document is updated (linear, nn)
• Not Applicable (i.e., the PR does not include API changes.)

Test quickstart & API (bash tests/docs/test_changed_document.sh)

If any APIs in quickstarts or tutorials are modified, please run this test to check if the current examples can run correctly after the modified APIs are released.

Not applicable; this PR does not modify quickstarts, tutorials, or documented APIs.