fix(provider): isolate unbound static credentials

[prekshivyas]

Summary

Withhold unbound static credentials at the gateway response boundary instead of sending incomplete binding metadata that makes a binding-capable supervisor reject every static credential in the snapshot.

Related Issue

No issue required: this is a localized fail-closed bug in the static credential binding path. A static credential without a binding cannot be safely projected by a binding-capable supervisor; the bug is that sending that invalid key causes the supervisor to revoke unrelated, correctly bound credentials too.

Design context: OpenShell #2510 states that an endpointless profile without a policy binding contributes non-secret configuration while OpenShell withholds its static credential keys and binding metadata. Its implementation comment explains that sending invalid metadata would let one rejected key revoke unrelated credentials. The released implementation covers the endpointless-profile case but misses the profileless None branch; this PR applies the same independent-withholding rationale to that missed case without asserting that #2510 already approved this exact implementation.

Changes

• Preserve provider resolution behavior and sanitize the completed environment only when the requesting supervisor advertises static-credential-binding support.
• Remove each unbound static key independently from the environment, expiry metadata, and static-key set before returning the snapshot.
• Retain bound static credentials and valid dynamic credentials when an unrelated static key is unbound.
• Preserve the existing legacy-supervisor behavior, which withholds all static credentials when binding metadata is unsupported.

Testing

• mise run pre-commit passes (mise is unavailable in this environment)
• Unit tests added/updated
• E2E tests added/updated (not applicable; this is covered at the provider-environment boundary)

Verification performed:

• cargo fmt --check
• cargo clippy -p openshell-server --features bundled-z3 --lib -- -D warnings
• cargo test -p openshell-server --features bundled-z3 --lib (1,408 passed, 0 failed, 7 ignored)
• GitHub Branch Checks on 3596078a: Rust passed on both linux-amd64-cpu8 and linux-arm64-cpu8; all 21 applicable checks succeeded.
• Regression coverage verifies that a profileless OPENAI_API_KEY is withheld while a bound GITHUB_TOKEN remains active, and that an unbound static key does not suppress an unrelated dynamic credential.

Checklist

• Follows Conventional Commits
• Commits are signed off (DCO)
• Architecture docs updated (not applicable; no public contract or architecture change)

[github-actions]

All contributors have signed the DCO ✍️ ✅
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[prekshivyas]

I have read the DCO document and I hereby sign the DCO.