feat: [FME-17261][FME-19301]: add experiment noun (CRUD) and experiment settings/alerts/results

[apetruccelli]

Description

• Tickets FME-17261 (experiment noun with full CRUD) and FME-19301 (experiment sub-resources: settings, alerts, results).
• experiment noun, full CRUD: list, get, create, update, delete against v4 (/fme/api/v4/experiments).
  • Experiments are scoped by parent — --parent-type FEATURE_FLAG|AI_CONFIG plus --parent-name/--parent-id — and by --env.
  • create builds the request from --start-at/--end-at/--baseline-treatment/--comparison-treatment/--key-metric/--supporting-metric/--description/--hypothesis, or accepts -f experiment.json as a full alternative.
  • update narrows the get-then-patch body to UpdateExperimentRequest's accepted fields, keeping read-only fields (id, type, parent, environment, timestamps) out of the PATCH — v4 rejects unknown properties.
• fme:metric_refs field type (modules/fme/metric_refs.go): keyMetrics/supportingMetrics read as [{id,name}] but write as plain metric id strings — an asymmetric read/write shape core's built-in set type can't handle since it assumes both shapes match. Normalize strips the display name down to a plain id string; Mutate adds/deletes/dedups on the normalized id list.
• owners is fully mutable, reusing the fme:owners field type already wired onto feature_flag/segment in feat: [FME-17257]: add owners/tags field types, flag_sets, and file (-f) support for feature_flag and segment #257 — no new Go code needed. --add/--del owners.user:<email|id> and owners.group:<identifier> work at both create and update.
• tags support (modules/fme/tags.go, new fme:tags field type): read shape is [{id,name}], write shape is [{name}] only. --add/--del tags.<name> work at update; list experiment --tag <name> filters by tag name.
• experiment:settings: get/update/delete the experiment's statistical settings (test type, significance threshold, MCC, minimum sample size, review period, variance reduction). Update is a get-then-patch against PATCH /fme/api/v4/experiments/{id}/settings; id/source are excluded from --set since they're server-derived and echoing them back 400s.
• experiment:alerts: get/update the experiment's alerting toggle (isEnabled). Update is a get-then-patch against PATCH /fme/api/v4/experiments/{id}/alerting.
• experiment:results: list metric results for an experiment (GET /fme/api/v4/experiments/{id}/metric-results), with an optional --metric-id filter mapped to the metric_ids query param. Null numeric/string fields render as empty rather than <nil>.

Commands

• harness create experiment <name> --parent-type feature_flag --parent-name my-flag --env <env-id> --start-at 2026-01-01T00:00:00Z --end-at 2026-02-01T00:00:00Z --baseline-treatment off --comparison-treatment on --add owners.user:<email>
• harness create experiment <name> -f experiment.json
• harness list experiment --parent-type FEATURE_FLAG --parent-name my-flag [--env <env-id>] [--status ACTIVE] [--tag <name>]
• harness get experiment <id>
• harness update experiment <id> --set description="new desc" --add key_metrics.<metric-id> --add owners.user:<email> --del owners.user:<id> --add tags.<name> --del tags.<name>
• harness delete experiment <id> --confirm
• fme get experiment:settings --id <experiment-id>
• fme update experiment:settings --id <experiment-id> --set <field>=<value>
• fme delete experiment:settings --id <experiment-id>
• fme get experiment:alerts --id <experiment-id>
• fme update experiment:alerts --id <experiment-id> --set is_enabled=<true|false>
• fme list experiment:results --id <experiment-id> [--metric-id <metric-id>]

Testing

• go build ./... and go test ./pkg/... ./modules/... pass on the branch rebased onto latest main.
• Unit tests in pkg/specloader/fme_spec_test.go cover all 5 experiment operations, owners add/del, the tag filter and tag mutations, and all six sub-resource commands (get/update/delete settings, get/update alerts, list results with and without --metric-id).
• Live-verified against qa0: create/list/get/update/delete round trip, owners add/del, key-metric add on update, read-only field exclusion from PATCH, tags add/del and list experiment --tag.
• Live-verified that --org/--project scope flags and --format json|table work for list/get experiment.

Acceptance Criteria

• All 5 experiment operations implemented (list, get, create, update, delete)
• --org/--project scope flags work
• --format json|table output works for list/get
• get/update/delete experiment:settings implemented against the v4 settings endpoint
• get/update experiment:alerts implemented against the v4 alerting endpoint
• list experiment:results implemented against the v4 metric-results endpoint, with --metric-id filter
• Null numeric/string result fields render as empty, not <nil>
• Unit tests for each operation

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