refactor alb mechanisms

[jranson]

This PR makes improvements to the ALB for performance and un-dry:

• Adds Pool() and HealthyTargets() functions to ALB Mechanism interface
• Exports consts for mechanism names and utilizes throughout
• Removes ResponseGate and ResponderClaim types in favor of a generic CaptureResponseWriter
• Refactors/simplifies all mechanisms to use CaptureResponseWriter

Also:

• Added bonus, revives the caller log entry field from trickster 1.x and earlier versions of trickster 2.x that had disappeared, like:
  time=2025-12-16T23:46:50.587335Z app=trickster level=warn event="configuration reload starting now" caller=pkg/daemon/daemon.go:132 source=sighup
• Updates the default HTTP/TCP backend timeouts as follows:
  • keep_alive_timeout: 300s -> 120s
  • timeout: 180s -> 60s
• Sets backend http clients' IdleConnTimeout value based on o.KeepAliveTimeout to close stale goroutines, and adds a max 10s TCP/HTTP/TLS connect/handshake timeout.

[coveralls]

Pull Request Test Coverage Report for Build 20318367811

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

• For more information on this, see Tracking coverage changes with pull request builds.
• To avoid this issue with future PRs, see these Recommended CI Configurations.
• For a quick fix, rebase this PR at GitHub. Your next report should be accurate.

Details

• 366 of 591 (61.93%) changed or added relevant lines in 37 files are covered.
• 12 unchanged lines in 6 files lost coverage.
• Overall coverage decreased (-0.4%) to 72.142%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
pkg/backends/alb/mech/ur/user_router.go	0	1	0.0%
pkg/backends/prometheus/model/alerts.go	23	24	95.83%
pkg/backends/prometheus/model/series.go	16	17	94.12%
pkg/backends/prometheus/model/vector.go	10	11	90.91%
pkg/daemon/setup/setup.go	0	1	0.0%
pkg/testutil/albpool/albpool.go	0	1	0.0%
pkg/backends/prometheus/model/labels.go	11	13	84.62%
pkg/proxy/paths/options/options.go	1	3	33.33%
pkg/backends/alb/mech/rr/round_robin.go	1	4	25.0%
pkg/backends/options/options.go	0	3	0.0%

Files with Coverage Reduction	New Missed Lines	%
pkg/backends/prometheus/model/alerts.go	1	96.49%
pkg/backends/prometheus/model/series.go	1	93.33%
pkg/backends/prometheus/model/vector.go	1	85.71%
pkg/proxy/response/merge/timeseries.go	1	0.0%
pkg/backends/alb/mech/tsm/time_series_merge.go	2	56.2%
pkg/proxy/response/merge/merge.go	6	0.0%

Totals
Change from base Build 20284111388:	-0.4%
Covered Lines:	14932
Relevant Lines:	20698

---

💛 - Coveralls