test: make unasserted lifecycle deprecations fail the test suite (opt-in)

[krlmlr]

Establishes the lifecycle baseline requested for the ellipsis migration,
at error strength, as an opt-in test-suite mode:
IGRAPH_LIFECYCLE_ERRORS=true turns it on,
and one job in the full rcc matrix runs the suite that way.

What actually governs verbosity in tests

Investigating the flip from warnings to errors surfaced the real mechanism:
testthat's 3rd edition already forces lifecycle_verbosity = "warning"
inside every test_that() block
(local_test_context() → local_reproducible_output(), hard-coded).
Two consequences:

• soft deprecations — including indirect calls that
  deprecate_soft() keeps silent for users — have always signalled
  as warnings inside this test suite;
• a setup-file options(lifecycle_verbosity = ...) can never escalate
  them: the per-test forcing overrides it again at the start of each test.
  (globalCallingHandlers() is no alternative either:
  R refuses to install global handlers from testthat's setup-file context.)

What this PR does

tests/testthat/setup-lifecycle.R follows the repo precedent for
nonstandard test behaviors — the IGRAPH_CHECK_RNG_STATE seed-leak check
in tests/testthat/setup.R (#2713, #2714):
gated behind IGRAPH_LIFECYCLE_ERRORS=true,
the setup file shadows test_that() for all test files
and forwards each block to testthat::test_that()
as a literal language object,
prepended with rlang::local_options(lifecycle_verbosity = "error").
The bump runs inside the test, after testthat has forced "warning",
so any deprecation no expectation asserts —
including indirect soft deprecations —
fails its test outright.
With the variable unset, the suite behaves exactly like vanilla
testthat edition 3.

The escape hatches stay intact,
and the baseline test in test-migration-fixture.R pins the regime
in both modes (warning when off, error when on):

• lifecycle::expect_deprecated() pins the verbosity back to "warning"
  internally — asserted deprecations keep working;
• snapshot tests record the warning text under
  rlang::local_options(lifecycle_verbosity = "warning");
• a test can opt out with
  rlang::local_options(lifecycle_verbosity = "quiet").

Nine existing snapshot tests exercise deprecated interfaces on purpose
(eigen_centrality(scale =), as.directed(), as.undirected(),
as_adjacency_matrix(attr =), as_biadjacency_matrix(attr =),
get.edge.ids(), positional mode in radius()/eccentricity(),
subgraph.edges()) and carry that verbosity pin —
correct under both regimes, their recorded snapshots are unchanged.

The full-matrix entry

The new .github/versions-matrix.R — the generic hook that
rcc-full already forwards into the custom before-install action
(env: ${{ matrix.env }}) — contributes one entry on R-release,
so the job shows up as e.g. rcc: ubuntu-26.04 (4.5) with lifecycle errors:

data.frame(
  os = "ubuntu-26.04",
  r = r_versions[[2]],
  env = "IGRAPH_LIFECYCLE_ERRORS=true",
  desc = "with lifecycle errors"
)

custom/before-install/action.yml gains the matching generic env input
and writes it to $GITHUB_ENV
(the same shape duckdb-r uses for its vendored-build and
engine-poisoning entries),
so the variable reaches the R session that runs the tests.
The smoke job and every other matrix entry stay on vanilla behavior.

Baseline result

Both modes are clean — the full suite, locally:

• gate off: FAIL 0 · WARN 0 · SKIP 6 · PASS 8215
• gate on: FAIL 0 · WARN 0 · SKIP 6 · PASS 8215

(the skips are the known sandbox network failure in test-foreign.R
and locally unavailable Suggests).
Package-internal code passes keyword-only arguments by name everywhere;
beyond the nine verbosity pins, the enforcement needed no fixes.

Note for review

The override shadows test_that() only for code testthat sources after
the setup files — i.e. every test file;
a fully qualified testthat::test_that() call would bypass it
(none exist in the suite).
Like the RNG-state override, it forwards the block as a literal language
object (no rlang::inject()),
so srcrefs (failure locations), skips, and snapshot lookup behave
normally — verified empirically.
Enable at most one of IGRAPH_LIFECYCLE_ERRORS and
IGRAPH_CHECK_RNG_STATE:
both shadow test_that(),
and the RNG override (sourced later) would win;
the baseline test fails loudly if both are set.

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