feat(engine): support getDayOfWeekEnum().getDisplayName() in Painless - #92

Summary

OpenSearch Dashboards' UBI sample index pattern defines a scripted field (day_of_week) using:

doc['timestamp'].value.getDayOfWeekEnum().getDisplayName(TextStyle.FULL, Locale.ROOT)

xerj's Painless subset only implemented the numeric getDayOfWeek() accessor. getDayOfWeekEnum() and .getDisplayName(...) were unsupported, so the script failed — and since scripted-field errors are dropped silently (no error surfaced to the client, the field is just omitted from fields), the "Searches by Day of Week" dashboard panel rendered empty with no visible cause.

How this was found

While live-verifying the UBI dashboards end-to-end (see #86–#89, #91), the "Searches by Day of Week" panel was empty. exists queries on day_of_week initially looked like a data gap, but that was the wrong test — day_of_week is an OSD index-pattern scripted field (computed by Painless at query time), not a stored field, so exists was never going to find it either way. Fetching the index-pattern saved object directly surfaced the actual script in its fieldFormatMap/fields JSON. Reproduced directly against xerj with a script_fields request using that exact script: the field was silently absent from the response.

Fix

• date_component: added a getDayOfWeekEnum case returning a title-cased day name ("Monday".."Sunday") instead of the existing numeric weekday.
• eval_member_value: added getDayOfWeekEnum to the date-accessor dispatch, and a getDisplayName passthrough on strings (the TextStyle/Locale arguments are accepted syntactically but not interpreted — xerj already assumes a single locale elsewhere).

Test plan

• cargo fmt --check
• cargo clippy -p xerj-engine -p xerj-query -p xerj-api --all-targets -- -D warnings
• cargo test -p xerj-engine -p xerj-query -p xerj-api --lib (xerj-api 94/94, xerj-query 131/131, xerj-engine 259/261 — the 2 failures are the pre-existing, unrelated snapshot_path_security_tests failures also present on upstream/main)
• 2 new regression tests: day_of_week_enum_display_name_returns_full_english_name, day_of_week_enum_display_name_covers_the_week (all 7 weekdays)
• Live verification against a rebuilt combined test binary (this fix cherry-picked onto the other open-PR fixes, fix(api): treat OpenSearch's flat_object as an alias for flattened #86/feat(query): add script query support #87/feat(engine): add local functions and lambdas to Painless #88/fix(query): resolve named ES date formats for ignore_malformed validation #89/fix(engine): parse_date_ms accepts no-colon numeric zone offsets #91) + the real UBI sample dataset: confirmed the exact scripted-field/terms-agg script OSD's day_of_week field uses now returns correct buckets (curl against _search and _msearch), and the "Searches by Day of Week" dashboard panel — previously "No results found" — now renders a populated donut chart with all 7 weekdays after a dashboard refresh.

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https://claude.ai/code/session_014PRCbyt7Y2HDyhG1tbQTeL