…4712)


### Copy of: #4700

### What does this PR do?
Centralizes `DD_API_KEY` access through `internalConfig.APIKey()` 
instead of reading the environment variable directly via `env.Get("DD_API_KEY")`.

## Changes

- **`internal/config/config.go`** — Add `APIKey()` method to internal config
- **`internal/config/config_test.go`** — Add test case for `APIKey()`
- **`ddtrace/tracer/option.go`** — Replace `env.Get("DD_API_KEY")` with `c.internalConfig.APIKey()`
- **`ddtrace/tracer/telemetry.go`** — Replace `env.Get("DD_API_KEY")` with `c.internalConfig.APIKey()`, remove unused `env` import

## Testing

```bash
cd dd-trace-go
go test ./internal/config -run '^TestAPIKey$' -count=1 -v
```

<details>
<summary>Result</summary>

=== RUN   TestAPIKey  
=== RUN   TestAPIKey/from_env  
=== RUN   TestAPIKey/default_empty_when_unset  
--- PASS: TestAPIKey (0.00s)  
    --- PASS: TestAPIKey/from_env (0.00s)  
    --- PASS: TestAPIKey/default_empty_when_unset (0.00s)  
PASS
ok      github.com/DataDog/dd-trace-go/v2/internal/config       0.836s  

</details>

```bash
go test ./ddtrace/tracer/... -run Telemetry -count=1 -v
go test ./internal/llmobs/... -count=1
go test ./ddtrace/tracer/... -count=1
```

<details>
<summary>Result</summary>

--- PASS: TestTelemetryEnabled (0.55s)
--- PASS: TestTelemetryEnabled/tracer_start (0.25s)
--- PASS: TestTelemetryEnabled/telemetry_customer_or_dynamic_rules (0.04s)
--- PASS: TestTelemetryEnabled/telemetry_local_rules (0.02s)
--- PASS: TestTelemetryEnabled/tracer_start_with_empty_rules (0.01s)
--- PASS: TestTelemetryEnabled/profiler_start,_tracer_start (0.20s)
--- PASS: TestTelemetryEnabled/orchestrion_telemetry (0.03s)
PASS
ok      github.com/DataDog/dd-trace-go/v2/ddtrace/tracer        1.923s

</details>



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### Motivation

- [https://datadoghq.atlassian.net/browse/APMAPI-1895](https://datadoghq.atlassian.net/browse/APMAPI-1895) 

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### Reviewer's Checklist
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- [ ] Changed code has unit tests for its functionality at or near 100% coverage.
- [ ] [System-Tests](https://github.com/DataDog/system-tests/) covering this feature have been added and enabled with the va.b.c-dev version tag.
- [ ] There is a benchmark for any new code, or changes to existing code.
- [ ] If this interacts with the agent in a new way, a system test has been added.
- [ ] New code is free of linting errors. You can check this by running `make lint` locally.
- [ ] New code doesn't break existing tests. You can check this by running `make test` locally.
- [ ] Add an appropriate team label so this PR gets put in the right place for the release notes.
- [ ] All generated files are up to date. You can check this by running `make generate` locally.
- [ ] Non-trivial go.mod changes, e.g. adding new modules, are reviewed by @DataDog/dd-trace-go-guild. Make sure all nested modules are up to date by running `make fix-modules` locally.

Unsure? Have a question? Request a review!


Co-authored-by: hannahkm <[email protected]>
Co-authored-by: eunskin <[email protected]>
Co-authored-by: mikayla.toffler <[email protected]>