fix(agent): sign in at the current host when the PDS cannot be discovered

[ewanc26]

What this changes

wf_agent_login_discovered (v0.38.0) returned an error when a handle could not be resolved to a PDS, so an account that publishes no PDS record could not sign in at all. It now logs in at the agent's current host, as it did before discovery existed. *out_pds stays NULL in that case.

Found by Cobalt's end-to-end test (cobalt#202), whose mock PDS does not serve handle resolution.

Verification

test_agent_login_discovered passes on the host build (argument checks; an unresolvable .invalid handle reports no PDS). The end-to-end path is exercised by Cobalt's e2e once this is in a release.

Docs

Header comment on wf_agent_login_discovered; CHANGELOG entry.

Changelog: CHANGELOG.md

Checklist

• CI is green (CI gate and flow / conventions)
• No secrets, tokens or signing keys in the diff
• Tests added or changed with the behaviour; none skipped, disabled or deleted

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