feat(eap): implement gen_ai attribute transformations

[constantinius]

Summary

Implements the two gen_ai attribute transformations introduced in sentry-conventions PR #465:

• gen_ai_request_messages_to_input_messages — transforms gen_ai.request.messages (with content field per message) into gen_ai.input.messages (with parts array)
• gen_ai_response_to_output_messages — combines gen_ai.response.text and gen_ai.response.tool_calls into a single gen_ai.output.messages assistant message with typed parts

These go beyond simple key renaming — they reshape attribute values to match the new schema.

How it works

Attributes with _status: "transform" in sentry-conventions produce WriteBehavior::CurrentName, so normalize_attribute_names leaves them alone. The dedicated transformation code in the new gen_ai_transform module handles the full move-and-reshape, called from normalize_ai as part of AI span normalization (processing mode only).

The gen_ai.request.messages, gen_ai.response.text, and gen_ai.response.tool_calls legacy aliases are removed from SpanV1 SpanData so these deprecated keys survive into SpanV2 attributes where the transformation can process them.

gen_ai.request.messages → gen_ai.input.messages

For each message object, the content field is converted to a parts array:

• String content "hello" → [{"type": "text", "content": "hello"}]
• Array content (already parts-like) → kept, with text copied to content on each part if missing
• Other content types (e.g., tool message objects) → left unchanged
• Non-JSON values → moved as-is (key rename only)

gen_ai.response.text + gen_ai.response.tool_calls → gen_ai.output.messages

gen_ai.response.text handles multiple formats:

• Plain string, JSON string, JSON array of strings, JSON object with content, JSON array of objects with content

Tool calls get "type": "tool_call" added. Both are combined into one assistant message: [{"role": "assistant", "parts": [...]}].

Changes

• relay-conventions/build/attributes.rs — add Transform variant to DeprecationStatus, producing WriteBehavior::CurrentName
• relay-event-normalization/src/eap/gen_ai_transform.rs — new module with transformation logic
• relay-event-normalization/src/eap/ai.rs — normalize_ai calls transform_gen_ai as its first step
• relay-event-schema/src/protocol/span.rs — remove legacy aliases for the three deprecated attributes
• relay-conventions/sentry-conventions — update submodule to PR fix(server): Allow multipart requests without trailing newline #465 branch

Tests

• 21 Rust unit tests covering all input formats, edge cases, and idempotency
• 4 Python integration tests covering both SpanV1 (transaction) and SpanV2 (direct ingestion) paths
• Updated existing test_ai_spans_example_transaction expectations

Contributes to TET-2587

[linear-code]

TET-2587