Decode email headers without injected spaces

Use email.header.make_header for MIME header decoding so adjacent encoded/plain header parts preserve RFC spacing, with regression coverage.

tests/test_mcp_email_decode_header_spaces.py

@@ -0,0 +1,34 @@
+"""mcp email server _decode_header must not inject spaces between parts.
+
+email.header.decode_header returns plain-text runs WITH their surrounding
+whitespace (e.g. (b"Re: ", None)), so joining parts with " " produced a
+double space after "Re:" on every non-ASCII subject, a spurious space in
+"Name <addr>" senders, and violated RFC 2047 6.2 which requires whitespace
+between two adjacent encoded-words to be dropped.
+"""
+import pytest
+
+pytest.importorskip("mcp")
+
+import mcp_servers.email_server as es
+
+
+def test_prefix_then_encoded_word_single_space():
+    assert es._decode_header("Re: =?utf-8?b?SsOzc2U=?=") == "Re: J\u00f3se"
+
+
+def test_encoded_word_then_plain_text():
+    assert es._decode_header("=?utf-8?b?SsOzc2U=?= Smith") == "J\u00f3se Smith"
+
+
+def test_adjacent_encoded_words_join_without_space():
+    out = es._decode_header("=?iso-8859-1?q?Caf=E9?= =?utf-8?b?5pel5pys?=")
+    assert out == "Caf\u00e9\u65e5\u672c"
+
+
+def test_plain_ascii_header_unchanged():
+    assert es._decode_header("Weekly report") == "Weekly report"
+
+
+def test_empty_header():
+    assert es._decode_header("") == ""