feat: Add native LangChain instrumentation support

[liustve]

Description of changes:

• Adds native OTel instrumentation for LangChain following OTel's Gen's AI Agent semantic conventions 1.39

• Instruments and wraps BaseCallbackManager.__init__ to inject an OpenTelemetry callback handler that captures spans for LLM calls, chains, tools, and agent.

• Add auto-wiring using OTel's entry points system, so applications using ADOT Python auto-instrumentation will automatically trace LangChain workflows without any code changes.

Testing

• Unit tests added
• Contract tests added
• Manual E2E test with a LangChain sample app, example spans shown below:

{
    "resource": {
        "attributes": {
            "aws.local.service": "test-agent",
            "aws.service.type": "gen_ai_agent",
            "telemetry.sdk.language": "python",
            "service.name": "test-agent",
            "telemetry.sdk.version": "1.39.1",
            "telemetry.auto.version": "0.14.2.dev0-aws",
            "telemetry.sdk.name": "opentelemetry"
        }
    },
    "scope": {
        "name": "amazon.opentelemetry.distro.instrumentation.langchain",
        "version": "0.14.2.dev0"
    },
    "traceId": "698a7f1a4b0cc40aec119f69c718c440",
    "spanId": "17fd622727f03922",
    "parentSpanId": "52efbf084ee9c4a7",
    "flags": 256,
    "name": "invoke_agent WeatherMathAgent",
    "kind": "INTERNAL",
    "startTimeUnixNano": 1770684186300891000,
    "endTimeUnixNano": 1770684189493051000,
    "durationNano": 3192160000,
    "attributes": {
        "aws.local.service": "test-agent",
        "gen_ai.operation.name": "invoke_agent",
        "gen_ai.agent.name": "WeatherMathAgent",
        "gen_ai.request.model": "anthropic.claude-3-haiku-20240307-v1:0",
        "PlatformType": "Generic",
        "gen_ai.provider.name": "aws.bedrock",
        "aws.local.environment": "generic:default"
    },
    "status": {
        "code": "UNSET"
    }
}

{
    "resource": {
        "attributes": {
            "aws.local.service": "test-agent",
            "aws.service.type": "gen_ai_agent",
            "telemetry.sdk.language": "python",
            "service.name": "test-agent",
            "telemetry.sdk.version": "1.39.1",
            "telemetry.auto.version": "0.14.2.dev0-aws",
            "telemetry.sdk.name": "opentelemetry"
        }
    },
    "scope": {
        "name": "amazon.opentelemetry.distro.instrumentation.langchain",
        "version": "0.14.2.dev0"
    },
    "traceId": "698a7f1a4b0cc40aec119f69c718c440",
    "spanId": "3ea1452ef68cbaf7",
    "parentSpanId": "56dfee092e1402c0",
    "flags": 256,
    "name": "chat anthropic.claude-3-haiku-20240307-v1:0",
    "kind": "INTERNAL",
    "startTimeUnixNano": 1770684188259781000,
    "endTimeUnixNano": 1770684189492810000,
    "durationNano": 1233029000,
    "attributes": {
        "aws.local.service": "test-agent",
        "gen_ai.operation.name": "chat",
        "gen_ai.usage.input_tokens": 573,
        "gen_ai.usage.output_tokens": 29,
        "gen_ai.request.model": "anthropic.claude-3-haiku-20240307-v1:0",
        "gen_ai.response.model": "anthropic.claude-3-haiku-20240307-v1:0",
        "PlatformType": "Generic",
        "gen_ai.provider.name": "aws.bedrock",
        "aws.local.environment": "generic:default"
    },
    "status": {
        "code": "UNSET"
    }
}

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