refactor(api): replace gorilla/mux with net/http across agent servers

[pgimalac]

What does this PR do?

Replaces github.com/gorilla/mux (and gorilla/handlers) with the standard library net/http across most of the agent's HTTP servers. Second step in the series to remove gorilla entirely — follow-up to #50275.

Not included: cmd/system-probe (module router needs an API redesign) and comp/core/gui (auth middleware scoping needs a design decision). Those are the last two remaining uses.

Motivation

gorilla/mux has been archived since 2023. Go 1.22+ net/http covers everything we use.

Describe how you validated your changes

Builds pass for agent, cluster-agent, security-agent, and process-agent. Unit tests pass. Pre-push linter and test hooks pass.

[dd-octo-sts]

Go Package Import Differences

Baseline: ed51e26
Comparison: dfd5a19

binary	os	arch	change
heroku-agent	linux	amd64	+1, -0 +github.com/DataDog/datadog-agent/pkg/api/middleware
cluster-agent	linux	amd64	+1, -2 +github.com/DataDog/datadog-agent/pkg/api/middleware -github.com/gorilla/handlers -github.com/gorilla/mux
cluster-agent	linux	arm64	+1, -2 +github.com/DataDog/datadog-agent/pkg/api/middleware -github.com/gorilla/handlers -github.com/gorilla/mux
cluster-agent-cloudfoundry	linux	amd64	+1, -3 +github.com/DataDog/datadog-agent/pkg/api/middleware -github.com/felixge/httpsnoop -github.com/gorilla/handlers -github.com/gorilla/mux
cluster-agent-cloudfoundry	linux	arm64	+1, -3 +github.com/DataDog/datadog-agent/pkg/api/middleware -github.com/felixge/httpsnoop -github.com/gorilla/handlers -github.com/gorilla/mux
dogstatsd	linux	amd64	+0, -1 -github.com/gorilla/mux
dogstatsd	linux	arm64	+0, -1 -github.com/gorilla/mux
process-agent	linux	amd64	+0, -1 -github.com/gorilla/mux
process-agent	linux	arm64	+0, -1 -github.com/gorilla/mux
process-agent	windows	amd64	+0, -1 -github.com/gorilla/mux
process-agent	darwin	amd64	+0, -1 -github.com/gorilla/mux
process-agent	darwin	arm64	+0, -1 -github.com/gorilla/mux
heroku-process-agent	linux	amd64	+0, -1 -github.com/gorilla/mux
security-agent	linux	amd64	+0, -1 -github.com/gorilla/mux
security-agent	linux	arm64	+0, -1 -github.com/gorilla/mux
security-agent	windows	amd64	+0, -1 -github.com/gorilla/mux
host-profiler	linux	amd64	+0, -1 -github.com/gorilla/mux
host-profiler	linux	arm64	+0, -1 -github.com/gorilla/mux
installer	linux	amd64	+1, -1 +github.com/DataDog/datadog-agent/pkg/api/middleware -github.com/gorilla/mux
installer	linux	arm64	+1, -1 +github.com/DataDog/datadog-agent/pkg/api/middleware -github.com/gorilla/mux
installer	windows	amd64	+1, -1 +github.com/DataDog/datadog-agent/pkg/api/middleware -github.com/gorilla/mux
privateactionrunner	linux	amd64	+0, -1 -github.com/gorilla/mux
privateactionrunner	linux	arm64	+0, -1 -github.com/gorilla/mux
privateactionrunner	windows	amd64	+0, -1 -github.com/gorilla/mux
privateactionrunner	darwin	amd64	+0, -1 -github.com/gorilla/mux
privateactionrunner	darwin	arm64	+0, -1 -github.com/gorilla/mux

[datadog-prod-us1-5]

🎯 Code Coverage (details)
• Patch Coverage: 32.80%
• Overall Coverage: 50.25% (-0.02%)

_{This comment will be updated automatically if new data arrives.
🔗 Commit SHA: dfd5a19 | Docs | Datadog PR Page | Give us feedback!}

[dd-octo-sts]

Files inventory check summary

File checks results against ancestor ed51e26c:

Results for datadog-agent_7.80.0~devel.git.512.dfd5a19.pipeline.111745096-1_amd64.deb:

No change detected

[dd-octo-sts]

Static quality checks

✅ Please find below the results from static quality gates
Comparison made with ancestor ed51e26
📊 Static Quality Gates Dashboard
🔗 SQG Job

Successful checks

Info

	Quality gate	Change	Size (prev → curr → max)
✅	agent_deb_amd64	-112.5 KiB (0.01% reduction)	742.253 → 742.143 → 750.310
✅	agent_deb_amd64_fips	-96.4 KiB (0.01% reduction)	700.315 → 700.221 → 702.690
✅	agent_msi	-104.14 KiB (0.02% reduction)	608.024 → 607.923 → 623.540
✅	agent_rpm_amd64	-112.5 KiB (0.01% reduction)	742.236 → 742.126 → 750.280
✅	agent_rpm_amd64_fips	-96.4 KiB (0.01% reduction)	700.299 → 700.205 → 702.670
✅	agent_rpm_arm64	-61.57 KiB (0.01% reduction)	720.159 → 720.099 → 724.050
✅	agent_rpm_arm64_fips	-93.5 KiB (0.01% reduction)	681.324 → 681.233 → 684.460
✅	agent_suse_amd64	-112.5 KiB (0.01% reduction)	742.236 → 742.126 → 750.280
✅	agent_suse_amd64_fips	-96.4 KiB (0.01% reduction)	700.299 → 700.205 → 702.670
✅	agent_suse_arm64	-61.57 KiB (0.01% reduction)	720.159 → 720.099 → 724.050
✅	agent_suse_arm64_fips	-93.5 KiB (0.01% reduction)	681.324 → 681.233 → 684.460
✅	docker_agent_amd64	-72.33 KiB (0.01% reduction)	802.491 → 802.420 → 805.870
✅	docker_agent_arm64	-61.41 KiB (0.01% reduction)	805.258 → 805.198 → 809.730
✅	docker_agent_jmx_amd64	-72.34 KiB (0.01% reduction)	993.411 → 993.340 → 996.590
✅	docker_agent_jmx_arm64	-61.41 KiB (0.01% reduction)	984.956 → 984.896 → 989.410
✅	docker_cluster_agent_amd64	-36.1 KiB (0.02% reduction)	206.607 → 206.571 → 207.600
✅	docker_host_profiler_amd64	-98.16 KiB (0.03% reduction)	301.579 → 301.484 → 315.800
✅	docker_host_profiler_arm64	-88.65 KiB (0.03% reduction)	313.059 → 312.972 → 327.400
✅	docker_dogstatsd_amd64	-36.16 KiB (0.09% reduction)	39.496 → 39.460 → 39.540
✅	dogstatsd_deb_amd64	-36.16 KiB (0.12% reduction)	30.149 → 30.114 → 30.770
✅	dogstatsd_deb_arm64	-32.12 KiB (0.11% reduction)	28.279 → 28.247 → 29.270
✅	dogstatsd_rpm_amd64	-36.16 KiB (0.12% reduction)	30.149 → 30.114 → 30.770
✅	dogstatsd_suse_amd64	-36.16 KiB (0.12% reduction)	30.149 → 30.114 → 30.770
✅	iot_agent_deb_amd64	+8.0 KiB (0.02% increase)	44.462 → 44.470 → 44.970
✅	iot_agent_deb_arm64	+4.0 KiB (0.01% increase)	41.442 → 41.446 → 42.560
✅	iot_agent_deb_armhf	+4.0 KiB (0.01% increase)	42.183 → 42.186 → 42.740
✅	iot_agent_rpm_amd64	+8.0 KiB (0.02% increase)	44.462 → 44.470 → 44.970
✅	iot_agent_suse_amd64	+8.0 KiB (0.02% increase)	44.462 → 44.470 → 44.970

5 successful checks with minimal change (< 2 KiB)

	Quality gate	Current Size
✅	agent_heroku_amd64	309.178 MiB
✅	docker_cluster_agent_arm64	220.634 MiB
✅	docker_cws_instrumentation_amd64	7.142 MiB
✅	docker_cws_instrumentation_arm64	6.689 MiB
✅	docker_dogstatsd_arm64	37.690 MiB