fix(tracing): report streaming errors to generation spans

[fcy222fcy]

Description

When a streaming LLM request fails, the Langfuse TraceGeneration span was still emitted with the successful-looking streaming_response value and without error metadata. This made failed generations appear successful.

This change captures interfaces.StreamEventError, records the error on the main streaming span, and adds metadata["error"] to the generation span. Failed streams now use explicit <error> or <redacted> response placeholders, while successful streams preserve the existing streaming_response behavior.

Fixes #328

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update

How Has This Been Tested?

The following tests and checks passed locally:

• go test -count=1 ./pkg/tracing
• go test -race -count=1 ./pkg/tracing
• go test -count=1 ./...
• go test -race -count=1 ./...
• go vet ./pkg/tracing
• golangci-lint run ./...
• gosec ./...
• gofmt and git diff --check

The regression tests cover both content policies, successful-stream compatibility, error metadata, and exception events on the main streaming span.

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules