caseworkctl source add should support non-empty flow-style accessProfiles

[jeremi]

Problem

caseworkctl source add now accepts the shipped empty flow form accessProfiles: [] and preserves a trailing comment when converting it to the block form needed for the generated reader profile. It still refuses a valid non-empty flow-style sequence such as accessProfiles: [{id: reader, ...}] because safely editing arbitrary flow YAML while preserving authored text and comments is a broader patching problem.

This is presentation-dependent behavior for valid BReg YAML, but it does not block the shipped starters or the tutorial added by #1016.

Acceptance criteria

• caseworkctl source add can add the generated reader profile when accessProfiles is a valid non-empty flow-style YAML sequence.
• Both one-line and multiline flow sequences accepted by the BReg parser are covered.
• Existing profiles, comments, and unrelated authored text are preserved, or the command clearly previews an intentional localized normalization before apply.
• The rendered candidate round-trips to exactly the expected parsed BReg project.
• Conflicting existing casework-reader profiles remain fail-closed.

Relationship

Follow-up to #1016. The common empty flow form is already handled there.