Change gen-ai attribute truncation from exempt to 256KB limit

[Copilot]

Description

Follow-up to #56914. Requirement changed: gen-ai semantic convention properties should be truncated to 256KB rather than being fully exempt from truncation.

Changes:

• Added GenAi_Properties_MaxValueLength constant (256 × 1024 = 262,144 chars) to SchemaConstants
• Renamed TruncationExemptProperties → GenAiProperties to reflect the updated semantics
• TraceHelper.cs / LogsHelper.cs: gen-ai properties now truncate to 256KB instead of bypassing truncation; non-gen-ai properties unchanged
• Renamed and updated test class (GenAiTruncationTests) with 300K-char payloads asserting truncation to 256KB

All 38 tests pass across net8.0, net9.0, net10.0.

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[Copilot]

Pull request overview

Updates Azure Monitor OpenTelemetry Exporter’s handling of GenAI semantic convention properties to truncate values at a higher 256KB limit (instead of being fully exempt), aligning with the updated requirement from #56914’s follow-up.

Changes:

• Introduced SchemaConstants.GenAi_Properties_MaxValueLength (256 × 1024) and renamed the key set to GenAiProperties.
• Updated trace/log attribute processing to apply the GenAI-specific truncation limit while keeping existing limits for non-GenAI properties.
• Updated/renamed truncation tests to validate GenAI truncation at 256KB using oversized payloads.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

File	Description
sdk/monitor/Azure.Monitor.OpenTelemetry.Exporter/src/Internals/SchemaConstants.cs	Adds the GenAI max value length constant and renames the GenAI key allowlist.
sdk/monitor/Azure.Monitor.OpenTelemetry.Exporter/src/Internals/TraceHelper.cs	Applies GenAI-specific truncation limit when adding span/custom dimension properties.
sdk/monitor/Azure.Monitor.OpenTelemetry.Exporter/src/Internals/LogsHelper.cs	Applies GenAI-specific truncation limit for log attributes and scope properties.
sdk/monitor/Azure.Monitor.OpenTelemetry.Exporter/tests/Azure.Monitor.OpenTelemetry.Exporter.Tests/GenAiTruncationTests.cs	Updates tests to assert truncation to the new GenAI limit across traces and logs.

Comments suppressed due to low confidence (1)

sdk/monitor/Azure.Monitor.OpenTelemetry.Exporter/tests/Azure.Monitor.OpenTelemetry.Exporter.Tests/GenAiTruncationTests.cs:26

• LargePayloadLength is hardcoded to 300,000. Since the truncation limit is now a const (SchemaConstants.GenAi_Properties_MaxValueLength), consider defining the test payload length relative to it (e.g., limit + some delta) to prevent the test from silently becoming non-exercising if the limit changes again.

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[harsimar]

/check-enforcer override