Check key and value validity in PbMap

[osa1]

Currently PbMap does not check ranges of values added as keys or values.

This allows serializing and deserializing a message with a map and getting a
different map back, because PbMap allows adding values that are out of range
(e.g. an integer larger than 32-bit range as a sfixed32 value) but when
serializing it truncates the value.

Update PbMap to use the same validation functions as PbList when adding
elements. Both keys and values are checked.

Fixes #1065.

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cl/836607306

[github-actions]

Package publishing

Package	Version	Status	Publish tag (post-merge)
package:protobuf	6.0.0	ready to publish	protobuf-v6.0.0
package:protoc_plugin	25.0.0	ready to publish	protoc_plugin-v25.0.0

Documentation at https://github.com/dart-lang/ecosystem/wiki/Publishing-automation.