Require explicit confirmation for no-initiator SAML captures

[abhiunix]

Summary

Removes implicit trust in the missing-initiator path of isAcceptableSamlPost. Today a SAMLResponse POST with details.initiator === undefined is treated as trusted, and a single-role assertion is auto-assumed and cached with no user interaction. This PR requires explicit confirmation for any capture that does not come from a trusted Okta initiator.

• Trusted Okta initiator → may auto-assume a single role (unchanged).
• Missing/untrusted initiator → capture is parked as pending (needsConfirm=true); the popup shows a confirm/assume step even for a single role.
• The existing selectRole path still validates the chosen roleArn against the stored assertion before calling STS, so no new trust is introduced.

Impact

This extension materializes AWS credentials in the browser that the normal console SAML flow never exposes to the client. The capture trigger is therefore a security boundary. Because a legitimate IdP auto-submit form (no-referrer, main_frame, no initiator) is indistinguishable from a replayed or forged top-level POST, the tabId>=0 / main_frame checks do not separate attacker from IdP. STS's signature check prevents forging an assertion but does not prevent replaying a still-valid one.

Attack scenarios

1. Assertion replay on a shared machine — a SAMLResponse captured from logs/history/a prior session is replayed via a top-level POST within its validity window; the extension auto-mints up-to-12h credentials that the attacker reads from the popup/storage.
2. Malicious/compromised tab triggers a top-level POST of a captured assertion with a stripped initiator; a single-role assertion is assumed with zero clicks.
3. Confused-deputy — a process that can issue the navigation but cannot itself call STS uses this extension as the deputy that turns an assertion into usable keys.

After this change, none of these mint credentials without an explicit user click.

Test plan

• Trusted Okta initiator, single role → auto-assumes (no regression)
• Trusted Okta initiator, multi role → picker (no regression)
• No-initiator, single role → NO auto-assume; confirm/assume button shown; STS only after click
• No-initiator, multi role → picker as before
• Replay a captured assertion with stripped initiator → nothing minted without a click
• Pending TTL (5 min) still evicts the unconfirmed capture
• Confirm still validates roleArn against the stored assertion

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