[ObjC] Fix nullability annotations on some GPB*Dictionary types.

As announced: https://protobuf.dev/news/2025-09-19/#objc

Correcting the some of the annotations as part of ObjC Major Version
bump coming in 2026Q1.

This is most likely to trip up Swift code calling it, but it could also
impact ObjC sources depending on the annotation enforcement the files
are compiled with.

Fixes #22403

PiperOrigin-RevId: 815778859

Author: thomasvl

objectivec/GPBDictionary.h

@@ -913,7 +913,7 @@ __attribute__((objc_subclassing_restricted))
  *
  * @return The object if found, nil otherwise.
  **/
-- (ObjectType)objectForKey:(uint32_t)key;
+- (nullable ObjectType)objectForKey:(uint32_t)key;
 
 /**
  * Enumerates the keys and values on this dictionary with the given block.
@@ -1843,7 +1843,7 @@ __attribute__((objc_subclassing_restricted))
  *
  * @return The object if found, nil otherwise.
  **/
-- (ObjectType)objectForKey:(int32_t)key;
+- (nullable ObjectType)objectForKey:(int32_t)key;
 
 /**
  * Enumerates the keys and values on this dictionary with the given block.
@@ -2773,7 +2773,7 @@ __attribute__((objc_subclassing_restricted))
  *
  * @return The object if found, nil otherwise.
  **/
-- (ObjectType)objectForKey:(uint64_t)key;
+- (nullable ObjectType)objectForKey:(uint64_t)key;
 
 /**
  * Enumerates the keys and values on this dictionary with the given block.
@@ -3703,7 +3703,7 @@ __attribute__((objc_subclassing_restricted))
  *
  * @return The object if found, nil otherwise.
  **/
-- (ObjectType)objectForKey:(int64_t)key;
+- (nullable ObjectType)objectForKey:(int64_t)key;
 
 /**
  * Enumerates the keys and values on this dictionary with the given block.
@@ -4633,7 +4633,7 @@ __attribute__((objc_subclassing_restricted))
  *
  * @return The object if found, nil otherwise.
  **/
-- (ObjectType)objectForKey:(BOOL)key;
+- (nullable ObjectType)objectForKey:(BOOL)key;
 
 /**
  * Enumerates the keys and values on this dictionary with the given block.
@@ -5556,7 +5556,7 @@ NS_ASSUME_NONNULL_END
 //% *
 //% * @return The object if found, nil otherwise.
 //% **/
-//%- (VALUE_TYPE)objectForKey:(KEY_TYPE)key;
+//%- (nullable VALUE_TYPE)objectForKey:(KEY_TYPE)key;
 //%PDDM-DEFINE VALUE_FOR_KEY_Enum(KEY_TYPE, VALUE_TYPE, VNAME)
 //%VALUE_FOR_KEY_POD(KEY_TYPE, VALUE_TYPE, VNAME)
 //%PDDM-DEFINE ARRAY_ARG_MODIFIERPOD()

objectivec/GPBDictionary.m

@@ -1179,7 +1179,7 @@ void GPBDictionaryReadEntry(id mapDictionary, GPBCodedInputStream *stream,
 //
 
 //%PDDM-DEFINE VALUE_FOR_KEY_OBJECT(KEY_TYPE, VALUE_NAME, VALUE_TYPE, KHELPER)
-//%- (VALUE_TYPE)objectForKey:(KEY_TYPE)key {
+//%- (nullable VALUE_TYPE)objectForKey:(KEY_TYPE)key {
 //%  VALUE_TYPE result = [_dictionary objectForKey:WRAPPED##KHELPER(key)];
 //%  return result;
 //%}
@@ -1312,7 +1312,7 @@ void GPBDictionaryReadEntry(id mapDictionary, GPBCodedInputStream *stream,
 //%PDDM-DEFINE BOOL_DICT_HASOBJECT(IDX, REF)
 //%REF##_values[IDX] != nil
 //%PDDM-DEFINE BOOL_VALUE_FOR_KEY_OBJECT(VALUE_NAME, VALUE_TYPE)
-//%- (VALUE_TYPE)objectForKey:(BOOL)key {
+//%- (nullable VALUE_TYPE)objectForKey:(BOOL)key {
 //%  return _values[key ? 1 : 0];
 //%}
 //%PDDM-DEFINE BOOL_SET_GPBVALUE_FOR_KEY_OBJECT(VALUE_NAME, VALUE_TYPE, VisP)
@@ -3063,7 +3063,7 @@ - (void)enumerateForTextFormat:(void (NS_NOESCAPE ^)(id keyObj, id valueObj))blo
   }];
 }
 
-- (id)objectForKey:(uint32_t)key {
+- (nullable id)objectForKey:(uint32_t)key {
   id result = [_dictionary objectForKey:@(key)];
   return result;
 }
@@ -4794,7 +4794,7 @@ - (void)enumerateForTextFormat:(void (NS_NOESCAPE ^)(id keyObj, id valueObj))blo
   }];
 }
 
-- (id)objectForKey:(int32_t)key {
+- (nullable id)objectForKey:(int32_t)key {
   id result = [_dictionary objectForKey:@(key)];
   return result;
 }
@@ -6525,7 +6525,7 @@ - (void)enumerateForTextFormat:(void (NS_NOESCAPE ^)(id keyObj, id valueObj))blo
   }];
 }
 
-- (id)objectForKey:(uint64_t)key {
+- (nullable id)objectForKey:(uint64_t)key {
   id result = [_dictionary objectForKey:@(key)];
   return result;
 }
@@ -8256,7 +8256,7 @@ - (void)enumerateForTextFormat:(void (NS_NOESCAPE ^)(id keyObj, id valueObj))blo
   }];
 }
 
-- (id)objectForKey:(int64_t)key {
+- (nullable id)objectForKey:(int64_t)key {
   id result = [_dictionary objectForKey:@(key)];
   return result;
 }
@@ -11438,7 +11438,7 @@ - (NSUInteger)count {
   return ((_values[0] != nil) ? 1 : 0) + ((_values[1] != nil) ? 1 : 0);
 }
 
-- (id)objectForKey:(BOOL)key {
+- (nullable id)objectForKey:(BOOL)key {
   return _values[key ? 1 : 0];
 }