[ML Inference Search Processors] Always return list when using dollar symbol in input_maps

[mingshl]

Description

fixing #2974

by adding a helpful method getMappedInputFromObject when reading search query/ search response _source document,

 * Retrieves the mapped input value from the object based on the provided field name.
 * when the field name start with a dollar symbol, which starts all path expressions in JSONPATH notation,
 * default to always return lists
 * otherwise, return the original format of elements retrieving from the fieldName

For example,

in search processors:

for the same item index, if configuring input_map as {"input": "$.item.text"},

the model input will be in list representation.

{
  "input": ["red shoes"]
}

for the same item index, if configuring input_map as {"input": "item.text"},

the model input will be in list representation.

{
  "input": "red shoes"
}

Check List

• New functionality includes testing.
• New functionality has been documented.
• API changes companion pull request created.
• Commits are signed per the DCO using --signoff.
• Public documentation issue/PR created.

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[mingshl]

rerunning flaky tests

RestConnectorToolIT > testConnectorToolInFlowAgent STANDARD_ERROR
    REPRODUCE WITH: ./gradlew ':opensearch-ml-plugin:integTest' --tests "org.opensearch.ml.rest.RestConnectorToolIT.testConnectorToolInFlowAgent" -Dtests.seed=FF62341AF71E5511 -Dtests.security.manager=false -Dtests.locale=cv-Cyrl-RU -Dtests.timezone=America/Thunder_Bay -Druntime.java=21


RestConnectorToolIT > testConnectorToolInFlowAgent FAILED
REPRODUCE WITH: ./gradlew ':opensearch-ml-plugin:integTest' --tests "org.opensearch.ml.rest.RestConnectorToolIT.testConnectorToolInFlowAgent" -Dtests.seed=FF62341AF71E5511 -Dtests.security.manager=false -Dtests.locale=cv-Cyrl-RU -Dtests.timezone=America/Thunder_Bay -Druntime.java=21
    org.opensearch.client.ResponseException: method [POST], host [http://[::1]:42541], URI [/_plugins/_ml/agents/2WmEIJIBugkMbA_Nmc3O/_execute], status line [HTTP/1.1 500 Internal Server Error]
    {"status":500,"error":{"type":"OpenSearchStatusException","reason":"System Error","details":"Error communicating with remote model: bedrock-runtime.us-west-2.amazonaws.com: Name or service not known"}}
        at __randomizedtesting.SeedInfo.seed([FF62341AF71E5511:DAC9050B68D574D9]:0)
        at app//org.opensearch.client.RestClient.convertResponse(RestClient.java:501)
        at app//org.opensearch.client.RestClient.performRequest(RestClient.java:384)
        at app//org.opensearch.client.RestClient.performRequest(RestClient.java:359)
        at app//org.opensearch.ml.utils.TestHelper.makeRequest(TestHelper.java:182)
        at app//org.opensearch.ml.utils.TestHelper.makeRequest(TestHelper.java:155)
        at app//org.opensearch.ml.utils.TestHelper.makeRequest(TestHelper.java:144)
        at app//org.opensearch.ml.rest.RestBaseAgentToolsIT.executeAgent(RestBaseAgentToolsIT.java:84)
        at app//org.opensearch.ml.rest.RestConnectorToolIT.testConnectorToolInFlowAgent(RestConnectorToolIT.java:140)

RestCon

[mingshl]

after considering standardization across ml commons repo, taking another fix #2985 closing this PR