Enable service.profilesSupport feature gate so profiles pipelines can start

**Is your feature request related to a problem? Please describe.**

Configuring a profiles pipeline makes the agent fail to start:

pipeline "profiles": profiling signal support is at alpha level, gated under the "service.profilesSupport" feature gate

Profiles are gated behind `service.profilesSupport`, which is alpha and off by
default in the collector the agent embeds. The agent builds the collector's
arguments itself and only passes `--config=`, so there is no supported way for an
operator to enable the gate. Profiles are unreachable from the agent today.

**Describe the solution you'd like**

Have the agent enable `service.profilesSupport` by default when it starts the
collector, so profiles pipelines work without operators needing to know the gate
exists.

**Describe alternatives you've considered**

Exposing a general `--feature-gates` passthrough in the agent config, or an
environment-variable opt-in. Both leave profiles off by default and require
operators to know which gate to name.

**Additional context**

I have a change ready and would like to submit a PR for this.

Start by locating where the agent constructs the embedded collector's startup arguments and currently passes only --config=. Trace the profiles pipeline startup path and add coverage for the feature-gate behavior; done means a profiles pipeline starts without the reported service.profilesSupport error.

Written by the indexing model from the issue text.