Fall back to English for messages with damaged format placeholders

[shashb27]

Refs #14664.

Problem

As detailed in #14664, some translated catalogues (e.g. el, fa) contain msgstrs with damaged str.format placeholders:

>>> import gettext
>>> t = gettext.translation('sphinx', 'sphinx/locale', languages=['el'])
>>> t.gettext("The config value `{name}' has type `{current.__name__}', "
...           "defaults to `{default.__name__}'.").format(name='x', current=int, default=str)
KeyError: 'current__name__'

Six call sites format translated text eagerly, so a single damaged placeholder in any locale kills the build with a KeyError that surfaces as an ExtensionError blaming an extension (sphinx/config.py x3, sphinx/_cli/__init__.py x2, sphinx/transforms/i18n.py).

Approach

Add sphinx.locale.safe_format: pass the untranslated message and the format arguments; it formats the translation and falls back to the original English message when the translation is damaged.

Damage detection compares the set of str.format field names between the msgid and its translation, plus a try/except around the format call:

• Renamed or added placeholders (e.g. {current.__name__} -> {current__name__}) and dropped placeholders are both caught. A plain try/except alone cannot catch dropped placeholders, because str.format silently ignores surplus arguments and would emit an incomplete message.
• Comparing sets (not sequences) keeps legitimate translator reordering ({1} ... {0}) working.
• When no translation is active, behaviour is identical to plain .format (the untranslated message formats itself).

Affected messages degrade to an English warning instead of a dead build. The catalogue data itself (the Transifex half of #14664) is not touched here — that needs someone with Transifex access, per the discussion in the issue.

Tests

• tests/test_locale.py: unit tests for the helper (healthy translation, renamed keyword, dropped positional, mixed corruption, reordered placeholders, other namespaces).
• tests/test_config/test_config.py: regression test driving a corrupted catalogue through check_confval_types, asserting the English fallback message.
• The full tests/test_intl/test_intl.py suite passes (61 tests) — the compare_references warnings now format through the helper.

[shashb27]

Heads-up on the red test matrix: the failing set here (test_viewcode*, test_default_highlight, test_html_code_role, the two test_additional_targets_* intl tests, plus the wrongenc.inc checkout encode errors) matches what every currently-open PR hits — the same jobs on #14654 and #14662 fail with the identical test list, and the last green main.yml run on master is from Aug 11. So this looks like dependency drift in the CI environment rather than something from this branch.

For what it's worth, the suites covering the code this branch touches pass in CI (test_config shows green in the matrix) and locally: tests/test_locale.py, tests/test_config/test_config.py, tests/test_command_line.py, and the full tests/test_intl/test_intl.py (61/61).