feature(container): Add claude vertex auth forwarding with GCP credential support

[CharlyRipp]

Description

Auto-forward Vertex AI and API provider environment variables into sandboxed containers, and auto-mount GCP Application Default Credentials when Vertex is enabled.

Closes #952

What changed

1. src/session/environment.rs: New AUTO_FORWARD_API_ENV_VARS constant that auto-forwards ANTHROPIC_API_KEY, ANTHROPIC_VERTEX_PROJECT_ID, ANTHROPIC_VERTEX_REGION, CLAUDE_CODE_USE_VERTEX, and CLOUD_ML_REGION when set on the host. Uses EnvEntry::Inherit (key-only in argv) to keep secrets out of process listings and logs.

2. src/session/container_config.rs: When CLAUDE_CODE_USE_VERTEX is set, mounts GCP credentials into the container at the well-known ADC path (/root/.config/gcloud/application_default_credentials.json). Supports both GOOGLE_APPLICATION_CREDENTIALS (custom path) and the default ADC location.

3. src/session/config.rs: Updated default_sandbox_environment() to include the new API vars so they appear in the TUI new-session env list.

How I tested

• Built with cargo build --release, created a sandboxed session
• Verified env vars appear inside the container (env | grep ANTHROPIC)
• Verified ADC file is mounted at the expected path
• cargo test passes (3 pre-existing container runtime failures unrelated to this change)
• cargo fmt --check and cargo clippy clean

PR Type

• New Feature
• Bug Fix
• Refactor
• Documentation
• Infrastructure / CI

Checklist

• I understand the code I am submitting
• New and existing tests pass
• Documentation was updated where necessary
• For UI changes: included screenshot or recording

AI Usage

• No AI was used
• AI was used for drafting/refactoring
• This is fully AI-generated

AI Model/Tool used: Claude Code

Any Additional AI Details you'd like to share:
Used Claude for codebase discovery, identifying the right env var names from official docs, edge case review, and code review of the changes. The feature idea, testing, and final decisions are mine.

• I am an AI Agent filling out this form (check box if true)

[Seluj78]

I mean, LGTM but I am not a rust expert :) And I cannot test

[njbrake]

Note: this comment was drafted by Claude via back-and-forth with @njbrake. The reasoning and decisions are his; the prose is Claude's.

Hey @CharlyRipp, thanks for this. Apologies for the unilateral push, but I went ahead and added a refactor commit on top of yours (7adfe5a3) to tighten scope before merge. If anything in here lands wrong with you, happy to revert or rework.

Quick summary of what changed:

1. Gated everything on CLAUDE_CODE_USE_VERTEX being set (and non-empty) on the host. The env var auto-forward now only runs when Vertex is enabled, so non-Vertex users see no behavior change. Empty string is treated as unset.
2. Gated the GCP cred mount on tool == "claude". CLAUDE_CODE_USE_VERTEX is Claude-specific, so no reason to expose GCP creds to opencode/codex/cursor sessions even if a user has the flag exported globally.
3. Reverted the change to default_sandbox_environment(). With (1) in place, the runtime auto-forward is the single mechanism, matching the DEFAULT_TERMINAL_ENV_VARS precedent. Adding the keys to the user-editable list created a confusing fake opt-out (user removes from the list, value still forwarded).
4. Added unit tests for the cred mount logic in container_config.rs: default ADC, custom GOOGLE_APPLICATION_CREDENTIALS, flag unset, flag empty, non-claude tool, missing ADC file.
5. Documented the auto-detect in docs/guides/sandbox.md.

The one place I'd value your input before merging: I dropped ANTHROPIC_API_KEY from the auto-forward list. My reasoning is that #952 is Vertex-specific (Vertex auth uses GCP, not the API key), and force-forwarding it would be a behavior change for users who happen to have it on their shell for unrelated reasons. They can still add it explicitly via sandbox.environment. But if you had a specific workflow in mind where both Vertex and the API key need to be in the container together, please push back, I'm happy to put it back.

The other four items are general cleanups I'd have asked for in any review. ANTHROPIC_API_KEY is the judgment call where your perspective matters most.

[CharlyRipp]

Perfect! Exactly things I was curious about and 💯 agree with.
The ANTHROPIC_API_KEY was definitely more forward thinking on other use-cases, outside of vertex.