supabase / supabase-flutter / 35877565309

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feat(supabase): add TraceContext.fromCarrier and TraceContext.w3c (#1871)

## What

`TraceContext` only had the default constructor taking three raw header
strings, so supplying a context meant hand-formatting a W3C
`traceparent`.

Two constructors, neither of which adds a dependency:

- `TraceContext.fromCarrier(Map<String, String> carrier)` reads the
headers an OpenTelemetry propagator injected into a carrier map. Names
match case-insensitively, empty values read as absent.
- `TraceContext.w3c(traceId:, spanId:, sampled:, tracestate:, baggage:)`
formats a `traceparent` for clients that expose ids rather than headers,
and throws `ArgumentError` when either id is not the hexadecimal shape
the specification requires.

The traceparent parsing the traced client did inline moves into
`trace_context_format.dart`, so the client and the constructors agree on
what a well-formed header is. Those helpers are `@internal`.

Parsing follows the specification's forward-compatibility rule: only
version `00` is held to exactly four fields, a higher version may append
fields that receivers parse past rather than reject, and version `ff` is
invalid. `formatTraceparent` lowercases the ids, since the grammar
admits lowercase hexadecimal only.

## Sentry does not need this

`formatAsW3CHeader(SentryTraceHeader)` is public API in `package:sentry`
(`lib/src/utils/tracing_utils.dart`, exported from `sentry.dart`), so
the Sentry recipe stays a one-liner:

```dart
traceContextProvider: () {
  final span = Sentry.getSpan();
  if (span == null) return null;
  return TraceContext(
    traceparent: formatAsW3CHeader(span.toSentryTrace()),
    baggage: span.toBaggageHeader()?.value,
  );
},
```

`toSentryTrace()` on its own yields the three-part `sentry-trace`
format, not W3C. `TraceContext.w3c` is kept for tracing clients with no
such conversion, and is easy to drop if you would rather not carry it.

## Scope

Additive. Both constructors are registered in `sdk-compliance.yaml`; th... (continued)

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