Return available_fields in patron blocking rule validation response (PP-3866)

[dbernstein]

Description

Returns the live patron data dictionary from the patron blocking rule
validation endpoint, and removes SIP-specific language from the
allowed-functions documentation. These changes are made in conjunction with
circulation-admin changes here.

• process_validate_patron_blocking_rule now returns a JSON body
  {"available_fields": live_values} on a successful 200 response,
  in addition to the existing error behaviour on 4xx. Previously the
  body was empty.
• docs/FUNCTIONS.md — replaced all references to "SIP2" and "SIP2
  server" with the protocol-agnostic "remote patron information call",
  since patron blocking rules are not exclusively a SIP2 feature.

Motivation and Context

The circulation-admin help modal (PP-3866) needs the live patron data
dictionary to display available template variable names and sample
values to administrators building blocking rules. Previously the
validation endpoint discarded this data after evaluating the rule
expression. Exposing it in the response body requires no additional
server round-trip.

The documentation update ensures that FUNCTIONS.md — which is
surfaced directly in the admin UI — does not mislead administrators
using non-SIP authentication providers that also support patron
blocking rules.

How Has This Been Tested?

• Confirmed via the running dev server that the validation endpoint
  returns the expected available_fields dictionary for a configured
  SIP2 service with test credentials.
• Existing controller and integration tests continue to pass (tox -e py312-docker -- --no-cov).
• These changes have been integration tested in my local environment
  with the related changes in circulation-admin

Checklist

• I have updated the documentation accordingly.
• All new and existing tests passed.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.27%. Comparing base (ed0f781) to head (9a840c4).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3179      +/-   ##
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- Coverage   93.28%   93.27%   -0.01%     
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  Files         496      496              
  Lines       46011    46012       +1     
  Branches     6302     6302              
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- Hits        42920    42919       -1     
- Misses       2004     2005       +1     
- Partials     1087     1088       +1     

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[jonathangreen]

Looks good, one minor comment.

[jonathangreen]

Minor: since you are changing the response shape, you should probably assert that available_fields is returned in the tests as well.