fix(compress): abort instead of dropping messages when summary LLM fails (#28102)

When auxiliary compression's summary generation returns None (aux model
errored, returned non-JSON, timed out, etc.) the compressor previously
still dropped every middle message between compress_start..compress_end
and replaced them with a static 'Summary generation was unavailable'
placeholder. The session kept going but the user silently lost N turns
of context for nothing.

New behavior: on summary failure, compress() aborts entirely — returns
the input messages unchanged and sets _last_compress_aborted=True. The
existing _summary_failure_cooldown_until gate (30-60s) keeps the aux
model from being burned on every turn. Auto-compress callers detect
the no-op (len(after) == len(before)) and stop looping. The chat is
'frozen' at its current size until the next /compress or /new.

Manual /compress (CLI + gateway) now passes force=True which clears
the cooldown so users can retry immediately after an auto-abort. If
the manual retry also fails, the user gets a visible warning telling
them nothing was dropped and how to retry.

- agent/context_compressor.py: compress() gains force= kwarg; failure
  branch sets _last_compress_aborted and returns messages unchanged
  instead of inserting placeholder.
- run_agent.py: _compress_context() detects abort, surfaces warning,
  skips session-rotation entirely, returns messages unchanged.
- cli.py + gateway/run.py: manual /compress paths pass force=True.
- gateway/run.py: hygiene + /compress handlers detect _last_compress_aborted
  and emit the new 'Compression aborted' warning (gateway.compress.aborted)
  instead of the old 'N historical messages were removed' message.
- locales/*.yaml: new gateway.compress.aborted key in all 16 locales.
- tests: updated to assert the abort contract (messages preserved,
  compression_count not incremented, abort flag set, no placeholder
  leaked). New test_force_true_bypasses_failure_cooldown covers the
  manual-retry path.

locales/ga.yaml

@@ -94,6 +94,7 @@ gateway:
     nothing_to_do:         "Níl aon rud le dlúthú fós (tá an traschríbhinn fós uile mar chomhthéacs cosanta)."
     focus_line:            "Fócas: \"{topic}\""
     summary_failed:        "⚠️ Theip ar ghiniúint achoimre ({error}). Baineadh {count} teachtaireacht stairiúil agus cuireadh ionadaí ina n-áit; níl an comhthéacs roimhe seo in-aisghabhála a thuilleadh. Smaoinigh ar an gcumraíocht auxiliary.compression a sheiceáil."
+    aborted:               "⚠️ Cuireadh deireadh leis an dlúthú ({error}). Níor baineadh aon teachtaireacht — tá an comhrá gan athrú. Rith /compress chun é a thriail arís, /reset le haghaidh seisiún glan, nó seiceáil do chumraíocht samhla auxiliary.compression."
     aux_failed:            "ℹ️ Theip ar an tsamhail dlúthúcháin chumraithe `{model}` ({error}). Aisghafa ag baint úsáide as do phríomhshamhail — tá an comhthéacs slán — ach b'fhéidir gur mhaith leat `auxiliary.compression.model` i config.yaml a sheiceáil."
     failed:                "Theip ar dhlúthú: {error}"