docs: document partial-failures.json for multi-version bootstrap

[LalatenduMohanty]

Description

--multiple-versions now writes a partial-failures.json artifact to the work directory when one or more package versions fail during bootstrap. This behavior was introduced by PR #1314 but is not documented for users.

The documentation gaps are:

• docs/how-tos/multiple-version-bootstrap.rst does not list the artifact under output files.
• docs/proposals/test-mode.md still says multi-version mode produces “Logs only”.

Requested changes

Document:

• When partial-failures.json is created.
• That it is removed on a subsequent run with no reportable failures.
• The report fields: name, version, phase, error_type, and message.
• That unresolved requirements use a null version.
• Behavior when --test-mode and --multiple-versions are combined.

Acceptance criteria

• The multi-version bootstrap how-to lists partial-failures.json.
• The report schema includes a brief example.
• The test-mode proposal no longer claims that multi-version mode produces logs only.
• The documentation identifies the release in which this behavior was added.