fix: restore double-brace escaping in instruction templates

Double braces (e.g. {{city}}) should produce literal {city} in the
rendered instruction, not trigger a session state lookup. This escaping
worked in v1.25.0 but regressed in v1.25.1 because the regex
r'{+[^{}]*}+' matches {{city}} and _replace_match strips all braces
before looking up the variable name.

The fix adds an early return in _replace_match: when the matched text
starts with {{ and ends with }}, peel one layer of braces and return
the literal string without any state/artifact lookup.

Fixes #4606

Author: stakeswky

src/google/adk/utils/instructions_utils.py

@@ -79,7 +79,11 @@ async def _async_sub(pattern, repl_async_fn, string) -> str:
     return ''.join(result)
 
   async def _replace_match(match) -> str:
-    var_name = match.group().lstrip('{').rstrip('}').strip()
+    raw = match.group()
+    # Double (or more) braces are escape sequences: {{x}} → {x}
+    if raw.startswith('{{') and raw.endswith('}}'):
+      return raw[1:-1]
+    var_name = raw.lstrip('{').rstrip('}').strip()
     optional = False
     if var_name.endswith('?'):
       optional = True

tests/unittests/utils/test_instructions_utils.py

@@ -267,3 +267,45 @@ async def test_inject_session_state_with_optional_missing_state_returns_empty():
       instruction_template, invocation_context
   )
   assert populated_instruction == "Optional value: "
+
+
+@pytest.mark.asyncio
+async def test_double_braces_escape_to_literal():
+  """Double braces {{x}} should produce literal {x}, not a state lookup."""
+  instruction_template = 'Generate a keyword like "roofing cost in {{city}}".'
+  invocation_context = await _create_test_readonly_context()
+
+  populated_instruction = await instructions_utils.inject_session_state(
+      instruction_template, invocation_context
+  )
+  assert populated_instruction == (
+      'Generate a keyword like "roofing cost in {city}".'
+  )
+
+
+@pytest.mark.asyncio
+async def test_double_braces_mixed_with_state_variable():
+  """Double braces should escape while single braces still resolve state."""
+  instruction_template = "Hello {user_name}, use {{placeholder}} in prompts."
+  invocation_context = await _create_test_readonly_context(
+      state={"user_name": "Alice"}
+  )
+
+  populated_instruction = await instructions_utils.inject_session_state(
+      instruction_template, invocation_context
+  )
+  assert populated_instruction == (
+      "Hello Alice, use {placeholder} in prompts."
+  )
+
+
+@pytest.mark.asyncio
+async def test_triple_braces_peel_one_layer():
+  """Triple braces {{{x}}} should peel one layer to {{x}}."""
+  instruction_template = "Escaped: {{{example}}}"
+  invocation_context = await _create_test_readonly_context()
+
+  populated_instruction = await instructions_utils.inject_session_state(
+      instruction_template, invocation_context
+  )
+  assert populated_instruction == "Escaped: {{example}}"