fix(provider): read sectors from the backend; clearer combobox and rate-limit failures

[saqibmanan]

Fixes the provider-smoke failures that have been red on every recent CI push and PR (for example #122 and #124: prv_006, 006b, 007, 011 and 012).

Root cause: stale test data, not load

The 2026-09-24 dev refresh from prod replaced the sector list. "Budgets" no longer exists on dev, whose 8 sectors are now Child Rights, Public Finance, Law And Justice, and so on. Eleven provider tests hardcoded it. The combobox never offered the option, so each test died with a blank TimeoutException on every rerun, which looked exactly like the old contention failures.

I checked the other hardcoded taxonomy values against dev (India/Assam, SDG 13, the Budget/Finance tags). All of them still exist.

Changes

• sector_name fixture (session scope): reads { sectors { name } } live from API_BASE_URL, the same pattern as backend_enum_labels. All 11 tests use it, and the stale SECTORS constants and aliases are removed from tests/data.
• Combobox timeouts name the problem. A new BasePage.wait_for_option() is used by select_combobox_option and the AI model picker. On timeout it raises No option 'X' after Ns. Options listed: [...], scoped to that input's own listbox so the Bhashini language options don't show up.
• go_to_metadata_tab uses click_until. After "Dataset created successfully" the form can re-render into "Loading" once more. test_prv_002 hit ElementClickIntercepted on the tab because of this.
• Failure hook flags a backend 429. At 1000 POST/hour per IP, a few suite runs are enough to throttle the backend, and the next failures then look like blank timeouts on the dashboard. The report now gets a "backend rate limit" section when the API answers 429 at failure time.

Evidence (headless, -n 3, against dev)

Run	Result
CI before the fix, -m smoke	5 failed, 7 passed, 10 rerun, 21 min
Sector fix, -m smoke	12 passed, 0 rerun, 8 min
Final branch, -m "smoke or functional"	17 passed, 1 xfailed, 1 xpassed, 0 rerun, 15 min

The new timeout message was checked by forcing the fixture to "Budgets": No option 'Budgets' after 30s. Options listed (filtered by the typed text): [...], on both the BasePage and AI model paths. The 429 section was checked while the backend was actually throttled.

Not changed, worth a look

• test_prv_008_org_add_charts XPASSes and test_prv_004_ind_add_charts still XFAILs. Both carry xfail("Charts feature isn't fully built yet"). Either charts now work for orgs and 004 has a real test bug, or 008 passes by accident. One XPASS isn't enough to drop the marker, and strict=False keeps it harmless.