…h OpenTelemetry GraphQL semantic conventions

Renames the Fusion gateway's `graphql.source.*` tags to `graphql.source_schema.*`,
removes the `graphql.source.id` attribute (not in spec, redundant with the
schema name), and drops `graphql.source.operation.type` since it duplicated
`graphql.operation.type`. Inherits the root-span `graphql.processing.type=request`
tag and the relocated `graphql.error` event from the shared diagnostics core.

Adds three new tests covering the OpenTelemetry GraphQL alignment behaviour:
the `MaxErrorEvents` cap on root-span error events with the full count still
recorded via `graphql.error.count`, and the `error.type` policy that prefers
`extensions.code` over the phase-appropriate fallback.

…parsing span

Adds the previously-omitted `graphql.processing.type=parse` tag to the
`GraphQL Document Parsing` span. The spec requires this attribute on every
processing-phase span; the prior comment about HTTP-middleware parsing no
longer applies because the parsing span only fires on the dedicated execution
pipeline phase.

…alignment

Updates the v15 to v16 migration guide to cover the renamed, removed, and
added telemetry attributes, the relocation of the `graphql.error` event to the
root span, the new `MaxErrorEvents` option, and the `error.type` value policy.

…ocument parsing span"

This reverts commit a154f1b.

…utes table

…no underlying exception

The graphql.error event was emitting fabricated values for exception.type,
exception.message, and exception.stacktrace when the GraphQL error had no
underlying CLR exception (e.g. exception.type would be "GRAPHQL_ERROR",
stacktrace would be empty). exception.* are OpenTelemetry standard
attributes describing a real runtime exception, so synthesizing them
misrepresents the data.

Now exception.{type,message,stacktrace} are only added when error.Exception
is not null. The GraphQL error message remains available via the
graphql.error.message attribute. The locked decision "always emit
exception.{type,message,stacktrace}" was about the WITH-exception case;
fabricating them when no exception is attached was an oversight.

Snapshots in HotChocolate.Diagnostics.Tests are regenerated to reflect the
removal. The v15 to v16 migration doc is updated to match.

…xception.* alignment

- error.type now reflects the GraphQL error code (HC0067, HC0020, etc.) or the
  underlying CLR exception type (System.Net.Http.HttpRequestException) instead
  of the EXECUTION_ERROR fallback in cases where a more specific value is
  available.
- exception.* attributes are removed from graphql.error events when no
  underlying CLR exception is attached.

…ing-span document.id

Removes the source-schema-request span that was emerging into a graphql.client-shaped
outbound span; we are not ready to commit to the spec's graphql.client surface yet.
Also drops graphql.document.id from the planning span (already carried by the parent
request span and not in the spec's plan-span attribute list).

…e fails

Previously the parsing span left Status as Unset and emitted no error.type when
the document failed to parse, even though the spec requires both. The span now
sets Status=Error and derives error.type from the first error (extensions.code
> exception type > GRAPHQL_PARSE_FAILED fallback).

…aphql.dataloader.*

Renames the openBatches and dispatchedBatches attributes on the BatchEvaluated
and BatchDispatched events to graphql.dataloader.batches.open and
graphql.dataloader.batches.dispatched, keeping every diagnostics attribute under
the graphql.* namespace and avoiding collisions with non-graphql instruments.
Documents both as HC extensions in the v16 migration guide.

GraphQL errors that originate from nested source-schema resolvers reach the
diagnostics layer without their field path because Fusion's source-schema
error remapping discards it. Tracked at #9624.

Inject IResolverContext into resolvers that throw GraphQLException and pass
the resolver path through ErrorBuilder.SetPath, so the test errors look like
realistic production errors that carry their location. Also fixes the deep
resolver test, whose query previously failed at validation because the path
ended on a Deeper field that did not declare causeFatalError.

… span error.type

The OpenTelemetry field execution span explicitly flips the precedence used by
phase spans: the exception type FQN should be preferred, with the GraphQL
extensions code as a secondary source. Adds a preferException flag to
SetGraphQLErrorType and uses it from the resolver listener so an
InvalidOperationException thrown from a resolver surfaces as
"System.InvalidOperationException" on the field span. Adds a regression test
covering a nullable resolver field that throws a plain .NET exception.