[nkurraDO]

Why

Hosted Agents resources are now regional (MARSOHS-2085). The API host a request reaches decides where a create is placed and where a resource can be found: api.digitalocean.com places in and serves the default region (ric1); api-<region>.digitalocean.com places in and serves that region. A resource is a 404 on every other region's host; there is no redirect, forwarding or fan-out. godo had no way to see or state a region.

What

• HostedAgentSession.Region, HostedAgentConfig.Region, HostedAgentConfigSummary.Region (json:"region,omitempty"), returned on create/get/list for teams on the regional routing rollout, omitted otherwise.
• HostedAgentSessionFromEnvironmentRequest.Region (omitempty): an optional placement check that must match the host called, else the server answers 4xx region_mismatch. Not sent when empty, so today's body is byte-for-byte unchanged. The YAML manifest create and the Environment create carry no region (the server rejects unknown body fields); placement is the host.
• Location (json:"-") on HostedAgentSession and HostedAgentConfig, lifted from the create response's Location header on every create method; the canonical URL on the owning region's host.
• HostedAgentsRegionalBaseURL(region): returns https://api-<region>.digitalocean.com/ in the form SetBaseURL accepts (empty region → default base URL). No new dialers: callers build a client per region.

c, _ := godo.New(httpClient, godo.SetBaseURL(godo.HostedAgentsRegionalBaseURL("mem1")))
sess, _, err := c.HostedAgents.CreateSessionFromEnvironment(ctx, &godo.HostedAgentSessionFromEnvironmentRequest{
    Name: "review-bot", ConfigID: cfg.ID, Region: "mem1",
})
// sess.Region == "mem1"; sess.Location is on api-mem1; keep using c for follow-ups.

Tests

• TestHostedAgents_CreateSessionFromEnvironment_RegionAndLocation: body region sent as given; Region, Location and X-DO-Region surface.
• TestHostedAgents_CreateSessionFromEnvironment_NoRegionIsOmitted: no region key when empty; no header → empty Location.
• TestHostedAgents_CreateEnvironment_RegionAndLocation, TestHostedAgents_ListEnvironments_Region: environment create has no body region; response region/Location surface on create and list.
• TestHostedAgentsRegionalBaseURL.

go vet ./... && go test . green. CHANGELOG left for the release PR per CONTRIBUTING.

[SSharma-10]

LGTM