fix(sources): fix unexpected timestamp field

[rwakulszowa]

When log namespacing is enabled, exec and http_server sources include a redundant timestamp field when decoding is enabled. When decoding is not enabled, they put the content under a "message" key, instead of directly in the root object.

In the case of exec, the issue was caused by passing a hardcoded LogNamespace value to the decoder. Fixed by passing the parsed value.

In the case of http_server, the decoder ignored the global LogNamespace. Fixed by passing the correctly parsed value to the decoder.

Fixes #20404.

[bits-bot]

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[rwakulszowa]

Tested manually with the following config (copied verbatim from #20404):

schema:
  log_namespace: true

sources:
  source_exec:
    type: exec
    command: ['echo', '{"log": "123"}']
    mode: scheduled
  source_http_server:
    type: http_server
    address: 0.0.0.0:80
  source_socket:
    type: socket
    address: 0.0.0.0:9000
    mode: tcp

sinks:
  sink_console:
    inputs:
      - source_*
    type: console
    encoding:
      codec: json

Please double check, though. I'm not a ddog / vector expert, so it won't hurt to be extra careful here.