tests: Add full feature and coverage tests for PEP 750 template strings.

Includes corresponding .exp files because this feature is only available in
Python 3.14+.

Tests for `!a` conversion specifier and space after `!` are not included
because they are not supported by MicroPython.

Signed-off-by: Koudai Aono <koxudaxi@gmail.com>
Signed-off-by: Damien George <damien@micropython.org>

tests/basics/string_tstring_basic1.py

@@ -0,0 +1,131 @@
+print("=== Parser error tests ===")
+try:
+    exec('t_empty = t"{}"')
+    print(f"Empty expr: {t_empty.interpolations[0].value}")
+except SyntaxError as e:
+    print(f"Empty expr: SyntaxError - {e}")
+
+# Whitespace in expression (Python semantics are to strip trailing)
+t_ws = t"{ 42 }"
+print(f"Whitespace: {str(t_ws)}")
+
+print("\n=== Error cases ===")
+try:
+    exec('t"{@}"')
+except SyntaxError:
+    print("Invalid syntax: SyntaxError")
+
+try:
+    long_expr = "x" * 10001
+    exec(f't"{{{long_expr}}}"')
+except (ValueError, SyntaxError, RuntimeError, NameError):
+    print("Long expr: Error")
+
+try:
+    exec('t"hello" "world"')
+except SyntaxError:
+    print("Mixed concat: SyntaxError")
+
+try:
+    exec('bt"test"')
+except SyntaxError:
+    print("bt prefix: SyntaxError")
+
+print("\n=== Escape sequence coverage tests ===")
+
+def expect_invalid_escape(label, expr):
+    try:
+        eval(expr)
+    except SyntaxError:
+        print(f"{label}: SyntaxError")
+    else:
+        print(f"{label}: ERROR (expected SyntaxError)")
+
+# Deprecated escape sequences (CPython warns, MicroPython accepts as literal)
+print(repr(t"\8"))
+print(repr(t"\9"))
+
+# Unknown escape chars
+print(repr(t"\z"))
+print(repr(t"\k"))
+
+# Invalid escapes
+expect_invalid_escape("Invalid \\x escape", 't"\\xGG"')
+expect_invalid_escape("Invalid \\u escape", 't"\\uGGGG"')
+expect_invalid_escape("Invalid \\U escape", 't"\\UGGGGGGGG"')
+
+# Unicode escapes (display format differs)
+print(repr(t"\x00\x01\xFF"))
+print(repr(t"\u0041"))
+print(repr(t"\u03B1"))
+print(repr(t"\u2764"))
+print(repr(t"\U00000041"))
+print(repr(t"\U0001F600"))
+print(repr(t"\x41\u0042\103"))
+
+# Unicode in triple-quoted strings
+unicode_test = t"""Unicode test:
+Emoji: {'\U0001f40d'}
+Special: {'\u03b1 \u03b2 \u03b3'}"""
+print(f"Unicode: {str(unicode_test)}")
+
+print("\n=== Trailing whitespace preservation (PEP 750) ===")
+x = 42
+tmpl_trail = t"{x   }"
+expr = tmpl_trail.interpolations[0].expression
+print(f"Expression with trailing spaces: |{expr}|")
+assert expr == "x", f"Expected 'x' but got '{expr}'"
+assert len(expr) == 1, f"Expected length 1 but got {len(expr)}"
+
+tmpl_both = t"{   x   }"
+expr2 = tmpl_both.interpolations[0].expression
+print(f"Expression with both spaces: |{expr2}|")
+assert expr2 == "   x", f"Expected '   x' but got '{expr2}'"
+assert len(expr2) == 4, f"Expected length 4 but got {len(expr2)}"
+
+tmpl_lead = t"{   x}"
+expr3 = tmpl_lead.interpolations[0].expression
+print(f"Expression with leading spaces: |{expr3}|")
+assert expr3 == "   x", f"Expected '   x' but got '{expr3}'"
+
+# Debug specifiers: leading space preserved, trailing space stripped from expression
+# PEP 750: "Whitespace is preserved in the debug specifier" (in strings part)
+z = 99
+t_debug4 = t"{z =}"
+assert t_debug4.strings == ("z =", "")
+assert t_debug4.interpolations[0].expression == "z"  # Trailing space stripped
+assert t_debug4.interpolations[0].conversion == "r"
+
+t_debug5 = t"{ z=}"
+assert t_debug5.strings == (" z=", "")
+assert t_debug5.interpolations[0].expression == " z"  # Leading space preserved
+assert t_debug5.interpolations[0].conversion == "r"
+
+t_debug6 = t"{ z =}"
+assert t_debug6.strings == (" z =", "")
+assert t_debug6.interpolations[0].expression == " z"  # Leading preserved, trailing stripped
+assert t_debug6.interpolations[0].conversion == "r"
+
+print("Trailing whitespace: PASS")
+
+print("\n=== Error message tests ===")
+try:
+    exec('t"}"')
+except SyntaxError:
+    print("Lone }} rejected (correct)")
+
+try:
+    exec('t"{"')
+except SyntaxError:
+    print("Unterminated {{ rejected (correct)")
+
+print("\n=== Triple quote empty interpolation ===")
+try:
+    exec('''empty_interp = t"""Start
+{}
+End"""''')
+    print(f"Empty interpolation: {str(empty_interp)}")
+except Exception as e:
+    print(f"Empty interpolation error: {type(e).__name__}: {e}")
+
+print("\nIncompatible tests completed!")