Update shared gen_ai attribute constants & utilities

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