feat: support multiple agents with shared instance-identity auth (#24325)

> This PR was authored by Mux on behalf of Mike.

## Summary

Adds support for multiple peer root workspace agents sharing the same
`auth_instance_id`, so AWS, Azure, and GCP instance-identity auth can
issue the correct session token for a selected agent instead of assuming
a
single root agent per instance.

## Problem

When a Terraform template attaches two or more `coder_agent` resources
(with `auth = "aws-instance-identity"`) to a single compute instance,
every agent shares the same cloud instance ID. The existing singular
lookup picks whichever agent was created most recently, silently
ignoring
the others.

## Solution

Introduce an optional pre-auth agent selector (`CODER_AGENT_NAME`) and
make the server-side lookup ambiguity-aware.

**Database layer:**
- `GetWorkspaceAgentsByInstanceID` (`:many`): returns all matching root
  agents for an instance ID.
- `GetWorkspaceAgentByInstanceIDAndName` (`:one`): returns the named
root
  agent for disambiguation.

**SDK and CLI:**
- `agent_name` field added to AWS, Azure, and GCP request structs
  (`omitempty` for backward compatibility).
- `CODER_AGENT_NAME` env var and `--agent-name` flag wired into the
agent
  bootstrap before instance-identity auth runs.

**Server handler (`handleAuthInstanceID`):**
- When `agent_name` is present: direct lookup by (instance ID, name).
- When absent: legacy lookup, then resource-scoped ambiguity check.
  Returns 409 with available agent names if multiple root agents match.
- Whitespace-only names are trimmed and treated as unspecified.
- Sub-agents remain excluded (`parent_id IS NULL` filter).

**Verification template:**
- `examples/templates/aws-multi-agent/` provisions one EC2 instance with
  two agents (`main` and `dev`), both using instance-identity auth with
  `CODER_AGENT_NAME` set in the cloud-init user data.

## Backward compatibility

Existing single-agent deployments work unchanged. The `agent_name` field
is optional with `omitempty`, and the unnamed path preserves today's
behavior when only one root agent matches.

coderd/database/querier.go

@@ -683,7 +683,6 @@ type sqlcQuerier interface {
 	GetWorkspaceACLByID(ctx context.Context, id uuid.UUID) (GetWorkspaceACLByIDRow, error)
 	GetWorkspaceAgentAndWorkspaceByID(ctx context.Context, id uuid.UUID) (GetWorkspaceAgentAndWorkspaceByIDRow, error)
 	GetWorkspaceAgentByID(ctx context.Context, id uuid.UUID) (WorkspaceAgent, error)
-	GetWorkspaceAgentByInstanceID(ctx context.Context, authInstanceID string) (WorkspaceAgent, error)
 	GetWorkspaceAgentDevcontainersByAgentID(ctx context.Context, workspaceAgentID uuid.UUID) ([]WorkspaceAgentDevcontainer, error)
 	GetWorkspaceAgentLifecycleStateByID(ctx context.Context, id uuid.UUID) (GetWorkspaceAgentLifecycleStateByIDRow, error)
 	GetWorkspaceAgentLogSourcesByAgentIDs(ctx context.Context, ids []uuid.UUID) ([]WorkspaceAgentLogSource, error)
@@ -697,6 +696,7 @@ type sqlcQuerier interface {
 	// `minute_buckets` could return 0 rows if there are no usage stats since `created_at`.
 	GetWorkspaceAgentUsageStats(ctx context.Context, createdAt time.Time) ([]GetWorkspaceAgentUsageStatsRow, error)
 	GetWorkspaceAgentUsageStatsAndLabels(ctx context.Context, createdAt time.Time) ([]GetWorkspaceAgentUsageStatsAndLabelsRow, error)
+	GetWorkspaceAgentsByInstanceID(ctx context.Context, authInstanceID string) ([]WorkspaceAgent, error)
 	GetWorkspaceAgentsByParentID(ctx context.Context, parentID uuid.UUID) ([]WorkspaceAgent, error)
 	GetWorkspaceAgentsByResourceIDs(ctx context.Context, ids []uuid.UUID) ([]WorkspaceAgent, error)
 	GetWorkspaceAgentsByWorkspaceAndBuildNumber(ctx context.Context, arg GetWorkspaceAgentsByWorkspaceAndBuildNumberParams) ([]WorkspaceAgent, error)