fix(compressor): strip _multimodal tool-result images in _strip_historical_media

[srojk34]

Problem

_strip_historical_media only searched for image-bearing user messages when
selecting the anchor. Sessions driven by browser_vision or computer_use
(native-vision fast path) accumulate screenshots exclusively in tool result
messages as _multimodal envelopes — their user turns carry plain text. Because
the anchor search never matched those tool results, no base-64 payload was ever
stripped. After two compression passes (e.g. 101 → 13 messages) the screenshots
were still present in the tail, pushing the outgoing payload above the provider's
HTTP 413 limit even though the estimated token count looked safe. Reported in #47339.

Fix

• Add _tool_result_has_media() to recognise _multimodal envelopes and
  OpenAI-style list content in tool-result messages.
• Add _strip_tool_result_media() to replace a _multimodal envelope with its
  text_summary string and to strip image parts from list-style tool results.
• Extend the anchor search in _strip_historical_media to also consider
  tool-result messages (user-image anchoring is preserved as-is).
• Strip tool-result images for all messages before the anchor alongside the
  existing user-image stripping.

Tests

TestStripHistoricalMediaToolResults (5 cases):

• _multimodal envelope in tool result before anchor → stripped to text_summary
• Most-recent tool result (the anchor itself) → left untouched
• Only one screenshot in session → nothing stripped (no-op, returns original list)
• Legacy user-image stripping still works unchanged
• OpenAI-style list content in tool results → image parts stripped

uv run --frozen python -m pytest tests/agent/test_context_compressor.py -x -q
101 passed in 6.11s

[alt-glitch]

Related: #47397, #42509, #37412 (all fix #47339 — context compression not evicting image/vision payloads, leading to HTTP 413).

This PR extends the merged _strip_historical_media fix from #27189, which only anchored on user-image messages and missed tool-result _multimodal envelopes accumulated by browser_vision / computer_use. Verified the gap is still live on main (no _tool_result_has_media/_strip_tool_result_media). Gap-fill of the incomplete merged fix → related, not duplicate. #47397 addresses the same issue at the 413 retry-recovery level instead of the compression root cause.

[srojk34]

Closing in favor of a more complete fix that strips all _multimodal tool-result images after compress(), not just pre-anchor ones.