feat(otel): make attribute truncation configurable

Adds `github.copilot.chat.otel.maxAttributeSizeChars` setting and
`COPILOT_OTEL_MAX_ATTRIBUTE_SIZE_CHARS` env var to control truncation
of free-form OTel content attributes (prompts, responses, tool
arguments/results, hook input/output).

Default is `0` (unlimited), matching the OTel spec's
`AttributeValueLengthLimit` default of `Infinity`. Users on backends
with per-attribute size limits can set a positive value to keep OTLP
batches under the backend cap.

Plumbs the resolved limit through every call site that previously hit a
hardcoded 64KB fallback. Drops the `DEFAULT_MAX_OTEL_ATTRIBUTE_LENGTH`
constant; `truncateForOTel`'s default arg is now `0` (unlimited).

Refs #299952