callbacks/otel: add OpenTelemetry callback handler

[pm32900]

Summary

Adds a new callbacks/otel package that implements the callbacks.Handler
interface and creates OpenTelemetry trace spans for LangChainGo lifecycle
events (LLM, chain, tool, retriever).

Motivation

LangChainGo has a clean callback interface but no first-class OpenTelemetry
integration. Production Go applications need standard observability — this
handler bridges that gap without touching any existing code or interfaces.

What's included

• callbacks/otel/handler.go — the Handler struct implementing callbacks.Handler
• callbacks/otel/options.go — functional options (WithTracerProvider, WithCaptureInput, WithCaptureOutput)
• callbacks/otel/handler_test.go — unit tests using tracetest.SpanRecorder (no external services)
• callbacks/otel/README.md — usage docs, privacy warning, instrumented events table
• examples/otel-callback-example/ — runnable example with stdout exporter

Design decisions

• Subpackage (callbacks/otel) so the go.opentelemetry.io/otel dependency
  is opt-in and does not affect users who don't need tracing
• Falls back to the global OTel TracerProvider if none is supplied — works
  with zero configuration
• No prompt/completion content captured by default — users must explicitly
  pass WithCaptureInput(true) / WithCaptureOutput(true)
• Streaming (HandleStreamingFunc) is intentionally a no-op — per-chunk spans
  would be extremely noisy
• GenAI semconv attribute keys (gen_ai.operation.name, gen_ai.usage.input_tokens, etc.)
  are local constants in one file, easy to update when otel/semconv stabilises them
• No existing interfaces or files were changed

Testing

go test ./callbacks/otel/...   # all pass
go test ./callbacks/...        # no regressions
go vet ./callbacks/otel/...    # clean

References
OpenTelemetry GenAI semantic conventions: https://opentelemetry.io/docs/specs/semconv/gen-ai/
Existing callbacks.Handler interface: callbacks/callbacks.go
Modelled after the existing LogHandler style in callbacks/log.go


### PR Checklist

- [x] Read the [Contributing documentation](https://github.com/tmc/langchaingo/blob/main/CONTRIBUTING.md).
- [x] Read the [Code of conduct documentation](https://github.com/tmc/langchaingo/blob/main/CODE_OF_CONDUCT.md).
- [x] Name your Pull Request title clearly, concisely, and prefixed with the name of the primarily affected package you changed according to [Good commit messages](https://go.dev/doc/contribute#commit_messages) (such as `memory: add interfaces for X, Y` or `util: add whizzbang helpers`).
- [x] Check that there isn't already a PR that solves the problem the same way to avoid creating a duplicate.
- [x] Provide a description in this PR that addresses **what** the PR is solving, or reference the issue that it solves (e.g. `Fixes #123`).
- [x] Describes the source of new concepts.
- [x] References existing implementations as appropriate.
- [x] Contains test coverage for new functions.
- [ ] Passes all [`golangci-lint`](https://golangci-lint.run/) checks.

[pm32900]

Hi! Just checking in to see if there's anything I can update or clarify to help move this PR forward. Happy to make any requested changes. Thanks!