fix: make streaming reasoning-tag filter case-insensitive

The streaming think-tag suppressors in cli.py (_stream_delta) and
gateway/stream_consumer.py (_filter_and_accumulate) matched tag names
with case-sensitive str.find(), so only the exact-case literals in the
tag tuples were caught. Mixed-case variants a model may emit — <Think>,
<ThInK>, <REASONING>, <Thought> — slipped through and leaked raw
reasoning into the user-visible stream.

Match against a lowercased view of the buffer with lowercased tag names
at all three sites (open-tag boundary search, partial-tag hold-back,
close-tag search) in both paths. Only KNOWN tag names are matched — no
substring matching — and the block-boundary gating that protects prose
mentions of <think> is preserved.

- 6 parametrized case-insensitive regression tests in each of
  tests/gateway/test_stream_consumer.py and
  tests/cli/test_stream_delta_think_tag.py.

Salvaged from PR #27289 by @YLChen-007.

tests/cli/test_stream_delta_think_tag.py

@@ -1,6 +1,9 @@
 """Tests for _stream_delta's handling of <think> tags in prose vs real reasoning blocks."""
 import sys
 import os
+
+import pytest
+
 sys.path.insert(0, os.path.join(os.path.dirname(__file__), "..", ".."))
 
 
@@ -110,6 +113,18 @@ class TestRealReasoningBlock:
         cli._stream_delta("   <think>")
         assert cli._in_reasoning_block
 
+    @pytest.mark.parametrize(
+        "tag",
+        ["THINK", "Think", "ThInK", "THOUGHT", "REASONING", "Thinking"],
+    )
+    def test_reasoning_tags_are_case_insensitive(self, tag):
+        cli = _make_cli_stub()
+        cli._stream_delta(f"<{tag}>hidden reasoning</{tag}>Visible answer")
+        assert not cli._in_reasoning_block
+        full = "".join(cli._emitted)
+        assert full == "Visible answer"
+        assert "hidden reasoning" not in full
+
 
 class TestFlushRecovery:
     """_flush_stream should recover content from false-positive reasoning blocks."""