test(mcp_server): cover single-pass prompt substitution

Direct unit coverage for _substitute (previously only exercised through the
subprocess round-trip on the happy path). Pins the literal-value contract:
an argument value containing {other} is not re-expanded, repeated and unknown
placeholders behave, and empty args leave the template untouched.

Author: Zawwarsami16

tests/test_mcp_server_substitute.py

@@ -0,0 +1,45 @@
+"""Unit coverage for mcp_server._substitute (prompts/get placeholder fill).
+
+These exercise the pure substitution helper directly — no hub or subprocess —
+so they pin the single-pass contract: an argument value is literal text, never
+re-scanned for further placeholders.
+"""
+
+from zhub.mcp_server import _substitute
+
+
+def test_basic_substitution():
+    assert _substitute("Hi {name}", {"name": "Sam"}) == "Hi Sam"
+
+
+def test_non_string_value_coerced():
+    assert _substitute("amount: {n}", {"n": 42}) == "amount: 42"
+
+
+def test_unknown_placeholder_left_intact():
+    assert _substitute("Hi {name} {missing}", {"name": "Sam"}) == "Hi Sam {missing}"
+
+
+def test_empty_args_returns_template_unchanged():
+    assert _substitute("No {vars} here", {}) == "No {vars} here"
+
+
+def test_repeated_placeholder_all_filled():
+    assert _substitute("{x}-{x}", {"x": "A"}) == "A-A"
+
+
+def test_value_containing_other_placeholder_is_literal():
+    # The argument order puts {text} before {style}; the naive replace-in-a-loop
+    # substituted {text} first, then re-scanned its inserted value and expanded
+    # the literal "{style}" inside it into the style argument. The value must
+    # stay verbatim.
+    out = _substitute(
+        "Text: {text}. Style: {style}",
+        {"text": "make it {style}", "style": "formal"},
+    )
+    assert out == "Text: make it {style}. Style: formal"
+
+
+def test_value_is_never_treated_as_template():
+    # {a}'s value is itself "{b}"; it must not be expanded into b's value.
+    assert _substitute("{a}", {"a": "{b}", "b": "Z"}) == "{b}"