Make msearch line format less lenient

[cbuescher]

A user recently discovered that we allow training closing brackets in _msearch request lines (see #61353).
It seems we parse only the first complete JSON object to a map, then discarding the rest of the line in MultiSearchRequest#readMultiLineFormat, allowing requests like the following:

GET my-index/_msearch
{"index": "my-index"}}}}
{"query" : {"match_all" : {}}}}}}
{"index": "my-index"}{ "this is" : "ignored as well"}
{"query" : {"match_all" : {}}}

While this is just a leniency that shouldn't affect the correct behaviour of the search, maybe we shouldn't allow this and error if we detect anything after parsing the initial json object.

[elasticmachine]

Pinging @elastic/es-search (:Search/Search)

[elasticsearchmachine]

Pinging @elastic/es-search (Team:Search)

[pranutyagi]

Hi, I am a first timer and want to work on this issue, can you please give more details on how to raise a pull request?

[Utsavk]

Hi @cbuescher , Can I give it try? It will be my first contribution.

[sujaysamanta]

Hello team,
This is my first time here and I want to contribute, is this issue still available to be picked up ..?