fix: honor parameter defaults in --use-parameter-defaults and SSH auto-start (#24591)

## Problem

The CLI does not honor `default` values on template parameters in two
ways:

1. **`--use-parameter-defaults` rejects empty-string defaults.** The
check `parameterValue != ""` means `default = ""` in Terraform falls
through to an interactive prompt. In CI this causes an EOF error.

2. **`--use-parameter-defaults` only exists on `coder create`.** The
`start`, `update`, and `restart` commands never wire it through. SSH
auto-start passes empty `workspaceParameterFlags{}`, so users SSH-ing
into a stopped workspace with new template parameters get stuck in an
interactive prompt they cannot complete.

## Fix

### 1. Fix empty-string default detection and expose flag on all
commands

Replace `parameterValue != ""` with a check based on `!tvp.Required`. A
parameter with `Required==false` always has a valid default in
Terraform, even if that default is `""`. Also respect CLI defaults
provided via `--parameter-default`.

Move `--use-parameter-defaults` from a standalone option on `create`
into the shared `workspaceParameterFlags` struct. This exposes the flag
(and `CODER_WORKSPACE_USE_PARAMETER_DEFAULTS`) on `start`, `update`, and
`restart` via `allOptions()`. Wire it through
`buildWorkspaceStartRequest` so the resolver receives it.

### 2. SSH auto-start always uses defaults

Set `useParameterDefaults: true` on both `startWorkspace` calls in the
SSH auto-start path (initial start and the forbidden/upgrade fallback).
SSH is non-interactive and should never prompt.

Fixes https://linear.app/codercom/issue/DEVEX-180
Fixes https://github.com/coder/coder/issues/22272

<details><summary>Implementation notes</summary>

### Scoping decisions

- **`--yes` does not imply `--use-parameter-defaults`**: Making `--yes`
auto-accept defaults exposes a validation gap in the dynamic parameter
path (client-side validation happens during prompting, and skipping
prompts bypasses it). This is deferred to a follow-up that also
addresses `codersdk.ValidateWorkspaceBuildParameter` integration in the
resolver. Tracked in PLAT-114.
- **Explicit overrides always win**: `--parameter`,
`--rich-parameter-file`, and `--preset` are resolved in stages 1-5 of
the resolver, before `resolveWithInput` runs. No change needed for
precedence.
- **`!tvp.Required` vs `parameterValue != ""`**: The `Required` field is
set by the Terraform provider based on whether a `default` is present.
This is the canonical signal for "has a default," not the string value
itself.

</details>

> Generated with [Coder Agents](https://coder.com/agents)

cli/parameter.go

@@ -24,11 +24,13 @@ type workspaceParameterFlags struct {
 	richParameterDefaults []string
 
 	promptRichParameters bool
+	useParameterDefaults bool
 }
 
 func (wpf *workspaceParameterFlags) allOptions() []serpent.Option {
 	options := append(wpf.cliEphemeralParameters(), wpf.cliParameters()...)
 	options = append(options, wpf.cliParameterDefaults()...)
+	options = append(options, wpf.useParameterDefaultsOption())
 	return append(options, wpf.alwaysPrompt())
 }
 
@@ -92,6 +94,15 @@ func (wpf *workspaceParameterFlags) cliParameterDefaults() []serpent.Option {
 	}
 }
 
+func (wpf *workspaceParameterFlags) useParameterDefaultsOption() serpent.Option {
+	return serpent.Option{
+		Flag:        "use-parameter-defaults",
+		Env:         "CODER_WORKSPACE_USE_PARAMETER_DEFAULTS",
+		Description: "Automatically accept parameter defaults when no value is provided.",
+		Value:       serpent.BoolOf(&wpf.useParameterDefaults),
+	}
+}
+
 func (wpf *workspaceParameterFlags) alwaysPrompt() serpent.Option {
 	return serpent.Option{
 		Flag:        "always-prompt",