feat: text helpers for report rendering

[Bhavikupadhyay]

Adds truncate_middle and pluralize for the PR-comment renderer. No tests included — that's the point: this PR is the live demonstration of coverage-agent generating them.

[github-actions]

coverage-agent found 2 gap(s), accepted 2 test(s) — scope: diff · model: groq/llama-3.3-70b-versatile

Gap	Kind	Result	Targets hit
coverage_agent/textutil.py:5->20	function	accepted	10/15
coverage_agent/textutil.py:23->28	function	accepted	4/5

2 accepted test files

tests/generated/test_coverageagent_truncate_middle.py

--- /dev/null
+++ b/tests/generated/test_coverageagent_truncate_middle.py
@@ -0,0 +1,33 @@
+import pytest
+from coverage_agent.textutil import truncate_middle
+
+def test_truncate_middle_short_text():
+    text = 'abc'
+    max_len = 5
+    assert truncate_middle(text, max_len) == text
+
+def test_truncate_middle_long_text():
+    text = 'abcdefghijklmnopqrstuvwxyz'
+    max_len = 10
+    result = truncate_middle(text, max_len)
+    assert len(result) <= max_len
+    assert result.startswith(text[:3]) and result.endswith(text[-3:])
+
+def test_truncate_middle_max_len_3():
+    text = 'abcdefghijklmnopqrstuvwxyz'
+    max_len = 3
+    result = truncate_middle(text, max_len)
+    assert len(result) <= max_len
+    assert result == text[:max_len]
+
+def test_truncate_middle_zero_max_len():
+    text = 'abcdefghijklmnopqrstuvwxyz'
+    max_len = 0
+    result = truncate_middle(text, max_len)
+    assert result == ""
+
+def test_truncate_middle_negative_max_len():
+    text = 'abcdefghijklmnopqrstuvwxyz'
+    max_len = -1
+    result = truncate_middle(text, max_len)
+    assert result == ""```

**`tests/generated/test_coverageagent_pluralize.py`**

```diff
--- /dev/null
+++ b/tests/generated/test_coverageagent_pluralize.py
@@ -0,0 +1,11 @@
+import pytest
+from coverage_agent.textutil import pluralize
+
+def test_pluralize_default_plural():
+    assert pluralize(2, 'cat') == '2 cats'
+    assert pluralize(3, 'dog') == '3 dogs'
+
+def test_pluralize_custom_plural():
+    assert pluralize(2, 'cat', 'cats') == '2 cats'
+    assert pluralize(3, 'dog', 'dogs') == '3 dogs'
+    assert pluralize(1, 'cat') == '1 cat'```

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