Copilot Chat otel.captureContent:false not honored on spans — chat spans carry full prompts/system instructions/responses (copilot-chat 0.57.0 in core)

Type: Bug

## `github.copilot.chat.otel.captureContent: false` is not honored on the span path — chat spans carry full prompts, system instructions, and responses

### Summary

With OTel emission enabled and `captureContent` explicitly `false` (the default), **span** attributes still include complete conversation content. The log and metric paths honor the setting; the span path does not. The setting's description says: "Capture input/output messages, system instructions, and tool definitions in OTel telemetry. **Contains potentially sensitive data.**" — so users reasonably rely on `false` to keep code and prompts out of their telemetry backend.

### Environment

- Copilot Chat **0.57.0** (built into VS Code core)
- VS Code stable, Windows 11 (10.0.26200)
- Settings:
```json
"github.copilot.chat.otel.enabled": true,
"github.copilot.chat.otel.exporterType": "otlp-grpc",
"github.copilot.chat.otel.protocol": "grpc",
"github.copilot.chat.otel.otlpEndpoint": "http://127.0.0.1:4317",
"github.copilot.chat.otel.captureContent": false
```
- No `COPILOT_OTEL_CAPTURE_CONTENT` environment variable set (no enterprise policy either).

### Repro

1. Configure as above, pointing at any OTLP collector (we used otelcol-contrib 0.156.0 with a file exporter).
2. Reload the window; run any chat request.
3. Inspect the exported `chat` spans.

### Observed

Every `chat` span carries, with `captureContent: false`:

- `copilot_chat.user_request` — the complete user prompt (6,505 chars in our repro)
- `gen_ai.system_instructions` — full system instructions (1,162 chars)
- `gen_ai.input.messages` — the complete message array
- `gen_ai.output.messages` — the full assistant response
- `gen_ai.tool.call.arguments`, `gen_ai.tool.call.result`, `gen_ai.tool.definitions`

Log records and metrics in the same capture correctly contain metadata only (token counts, model, timing) — the gating appears to be applied on those paths but missing on the span-building path.

### Expected

With `captureContent: false`, span attributes contain no message content, no system instructions, no tool arguments/results — matching the log/metric behavior and the setting's description.

### Impact

Anyone who enabled OTel spans while relying on `captureContent: false` is shipping full conversation content (which can include proprietary source code) to their telemetry backend without knowing it. For our part we've had to exclude span capture from all customer-adjacent environments until this is fixed — which is a pity, because the span path is the only place cache token detail (`gen_ai.usage.cache_read.input_tokens`) and `copilot_chat.copilot_usage_nano_aiu` are emitted, and those are exactly the attributes cost-observability setups want.

Happy to provide sanitized span samples on request.