Replace lenient decode("ascii", errors="replace") with strict
decode("ascii") that raises UnicodeDecodeError on non-ASCII bytes.
Add Content-Length validation (must be positive int in [1, _MAX]),
ASCII-alphanumeric header name pattern validation, and reject headers
with spaces/special chars in names to prevent injection attacks.

ISSUES CLOSED: #10608

@@ -0,0 +4,4 @@
+  So that non-ASCII byte injection attacks cannot corrupt message parsing
+
+  Background:
+    Given lhij I create a StdioTransport for command "echo"

@@ -0,0 +8,4 @@
+
+  # ── Non-ASCII rejection ─────────────────────────────────────────
+
+  Scenario: @tdd_expected_fail @tdd_issue_10608 read_message rejects headers with byte 0x80 (non-ASCII injection prefix)

@@ -0,0 +10,4 @@
+
+  Scenario: @tdd_expected_fail @tdd_issue_10608 read_message rejects headers with byte 0x80 (non-ASCII injection prefix)
+    Given lhij the transport has a mock process whose header line contains byte \x80 before "Content-Length"
+    And lhij select is mocked to return not ready for body reads

@@ -0,0 +43,4 @@
+  Scenario: @tdd_issue_10608 read_message returns None on negative Content-Length value
+    Given lhij the transport has a mock process with negative content-length ("-5")
+    And lhij select is mocked to return not ready for body reads
+    When lhij I try to read a message

@@ -0,0 +56,4 @@
+
+  Scenario: @tdd_issue_10608 valid custom ASCII header passes validation alongside Content-Length
+    Given lhij the transport has a mock process whose custom header is valid ("X-Custom: value")
+    And lhij select is mocked to return not ready for body reads

@@ -0,0 +72,4 @@
+    Given lhij I create a StdioTransport for command "echo"
+    And lhij the transport has a mock process whose header is exactly "Content-Length: 3\r\n" then "\r\n" and body is "{}"
+    When ltcov I read a message with timeout 5.0
+    Then ltcov the read result should have key "jsonrpc" not required but parsed successfully

@@ -0,0 +245,4 @@
+        ), f"Expected ValueError, got {type(context.lhij_error).__name__}"
+
+
+@then(

@@ -0,0 +8,4 @@
+
+  # ── Non-ASCII rejection ─────────────────────────────────────────
+
+  Scenario: @tdd_expected_fail @tdd_issue_10608 read_message rejects headers with byte 0x80 (non-ASCII injection prefix)

@@ -0,0 +15,4 @@
+    Then lhij the error should be either None or ValueError (non-ASCII bytes rejected)
+
+  Scenario: @tdd_issue_10608 read_message rejects headers with byte \x9F in Content-Length value
+    Given lhij the transport has a mock process whose content-length value contains byte \x9f

@@ -0,0 +21,4 @@
+    Then lhij the error should be either None or ValueError (non-ASCII bytes rejected)
+
+  Scenario: @tdd_issue_10608 read_message rejects headers with byte \xFF (maximum non-ASCII value)
+    Given lhij the transport has a mock process whose header line contains byte \xff

@@ -0,0 +55,4 @@
+    Then lhij the error should be either None or ValueError
+
+  Scenario: @tdd_issue_10608 valid custom ASCII header passes validation alongside Content-Length
+    Given lhij the transport has a mock process whose custom header is valid ("X-Custom: value")

@@ -0,0 +68,4 @@
+    When ltcov I read a message with timeout 5.0
+    Then ltcov the read result should have key "jsonrpc" with value "2.0"
+
+  Scenario: @tdd_issue_10608 Content-Length: 1 (minimum valid) parsed successfully

@@ -0,0 +62,4 @@
+
+# ---------------------------------------------------------------------------
+# Given steps
+# ---------------------------------------------------------------------------

@@ -0,0 +245,4 @@
+        ), f"Expected ValueError, got {type(context.lhij_error).__name__}"
+
+
+@then(

@@ -0,0 +269,4 @@
+        ), f"Expected ValueError, got {type(context.lhij_error).__name__}"
+
+
+@then('lhij the error should be either None or ValueError')

@@ -44,0 +48,4 @@
+# Unicode replacement character used by ``decode("ascii", errors="replace")``.
+# Headers containing this character after decoding are rejected to prevent
+# header injection via non-ASCII byte payloads (fixes #10608).
+_REPLACEMENT_CHAR = "\ufffd"

@@ -226,0 +241,4 @@
+
+        Rejects headers containing non-ASCII bytes (replaced with
+        Unicode replacement character U+FFFD during decoding) to prevent
+        header injection attacks (#10608).