Update shared gen_ai attribute constants & utilities

[andreiborza]

Update the shared attribute constants and utilities to align with RFC #153. This is the foundation for all per-provider work.

New constants to add

• gen_ai.provider.name (replaces gen_ai.system)
• gen_ai.tool.definitions (replaces gen_ai.request.available_tools)
• gen_ai.tool.call.arguments (replaces gen_ai.tool.input)
• gen_ai.tool.call.result (replaces gen_ai.tool.output)
• gen_ai.response.time_to_first_token
• gen_ai.embeddings.dimension.count
• gen_ai.usage.output_tokens.reasoning
• gen_ai.request.seed

Constants to deprecate

• gen_ai.system
• gen_ai.prompt
• gen_ai.request.available_tools
• gen_ai.response.text
• gen_ai.response.tool_calls
• gen_ai.tool.input
• gen_ai.tool.output
• gen_ai.usage.cache_creation_input_tokens
• gen_ai.usage.cache_read_input_tokens

Shared utilities

• Update setTokenUsageAttributes() to use new token attribute names
• Update message format utilities (content → parts)
• Update message truncation and media stripping helpers

[linear-code]

JS-2322