fix(compress): make abort-on-summary-failure opt-in via config flag (#28117)

PR #28102 made the summary-failure abort path the unconditional default,
changing established behavior. Gate it behind config.yaml flag
`compression.abort_on_summary_failure` (default False = historical
fallback-placeholder behavior).

- hermes_cli/config.py: new `compression.abort_on_summary_failure` key,
  default False, documented inline.
- agent/agent_init.py: read the flag from compression config and pass to
  ContextCompressor.
- agent/context_compressor.py: `__init__` accepts `abort_on_summary_failure`
  (default False). `compress()` failure branch gates the abort on the
  flag; when False, falls through to the restored legacy fallback path
  (static "summary unavailable" placeholder + drop middle window).
- tests: restore original fallback expectations as default; add new
  TestAbortOnSummaryFailure class for the opt-in mode.

Gateway/CLI plumbing (force=True on /compress, hygiene/handler abort
detection, locale `gateway.compress.aborted` key) from PR #28102 stays
intact — those paths only fire when `_last_compress_aborted` is True,
which now only happens when the flag is enabled.

agent/agent_init.py

@@ -1105,6 +1105,9 @@ def init_agent(
     compression_protect_first = max(
         0, int(_compression_cfg.get("protect_first_n", 3))
     )
+    compression_abort_on_summary_failure = str(
+        _compression_cfg.get("abort_on_summary_failure", False)
+    ).lower() in {"true", "1", "yes"}
 
     # Read optional explicit context_length override for the auxiliary
     # compression model. Custom endpoints often cannot report this via
@@ -1319,6 +1322,7 @@ def init_agent(
             config_context_length=_config_context_length,
             provider=agent.provider,
             api_mode=agent.api_mode,
+            abort_on_summary_failure=compression_abort_on_summary_failure,
         )
     agent.compression_enabled = compression_enabled