Publish end-of-support DBR runtimes still requested by clusters (12.2, 16.1)

[rugpanov]

Problem

VPEX E_ENV_UNSUPPORTED telemetry (an owned target with no environment published) shows a few end-of-support DBR runtime lines still requested by live clusters:

env_key	events	workspaces
dbr/12.2.x-scala2.12	8	2
dbr/16.1.x-scala2.12	1	1

(The louder E_ENV_UNSUPPORTED env_keys — dbr/18.x-scala2.13, dbr/16.4.x-scala2.13, dbr/16.4.x-cpu-ml-scala2.13 — are already fixed by #26 / #25.)

These EoS lines can't be published today because they are un-discoverable: they're dropped from the release-notes index (so discover_dbr never sees them) and skipped by dbr_point_releases' is_eos check, and the eos index page links none of them. There is no page that enumerates EoS runtimes.

Change

• sync.py — add DBR_EOS_PUBLISH (["12.2", "16.1"]): a narrow, telemetry-curated allowlist of EoS page slugs to publish anyway. Its comment carries the telemetry counts and the rule: add a slug only when telemetry shows its exact env_key getting E_ENV_UNSUPPORTED — a deliberate, scoped exception to the EoS-not-published policy.
• Factor sync_dbr's per-slug body into _sync_dbr_page(slug, point_release, umbrella_major) (mirrors the existing _sync_dbr_ml_page signature), and add sync_dbr_eos() that runs the allowlist through it directly — fetched by slug, so the EoS skip never applies. Each is a pre-18-style page (real minor in its title), so it publishes only its own <key_ver>.x folder, no bare-major umbrella. Wired into main().
• envgen.py — add 16.1.x-scala2.12 to DROP_BY_ENV. DBR 16.1 is Python 3.12 pinned to pandas 1.5.3 (no cp312 wheel), the same case as 16.4 / serverless-v3 (Drop pandas on DBR 16.4 + serverless-v3 (no cp312 wheel) #23). DBR 12.2 is Python 3.9 and keeps pandas~=1.4.2.

Scope: standard (non-ML) only — no EoS ML line has any E_ENV_UNSUPPORTED traffic. The allowlist pattern extends to sync_dbr_ml later if that changes.

Generated artifacts

• python/dbr/12.2.x-scala2.12/ (Python 3.9.21, 126 packages)
• python/dbr/16.1.x-scala2.12/ (Python 3.12.3, 164 packages)

Testing

• New SyncDbrEosTest: publishes an allowlisted line despite its (EoS) title; skips an allowlisted slug whose page is gone (fetch failure). Extended the envgen scoped-drop test to cover 16.1.x-scala2.12. Full suite: 45 tests pass.
• Both generated environments pass uv sync (verify_resolve.py, via the Databricks pypi proxy).

Relates to DECO-28292 (bug 3).

This pull request and its description were written by Isaac.