fix(cli): report cumulative total_tokens in stream-json/json output

[bzqzheng]

Problem

The CLI's "--json" and "--stream-json" output modes were reading session.total_tokens, which stores only the last turn's token usage. This caused complete.total_tokens to reset to the current turn's count on every chunk, rather than reporting the running session total (fixes #8871).

Root Cause

• update_session_metrics in crates/goose/src/agents/reply_parts.rs correctly computes accumulated_total_tokens by adding usage across turns.
• However, total_tokens is set to the current turn's usage.
• The CLI output code in crates/goose-cli/src/session/mod.rs reads session.total_tokens instead of session.accumulated_total_tokens.

Fix

Read session.accumulated_total_tokens in:

• JsonMetadata construction (--json mode)
• StreamEvent::Complete emission (--stream-json mode)
• get_total_token_usage() for consistency

Test

Added test_accumulated_total_tokens_across_multiple_turns which:

1. Creates a mock provider reporting 15 tokens per turn
2. Runs 3 reply turns
3. Asserts accumulated_total_tokens == 45 (cumulative)
4. Asserts total_tokens == 15 (last turn only)

Verification

cargo test --package goose test_accumulated_total_tokens_across_multiple_turns
cargo test --package goose-cli

[DOsinga]

Merged main, resolved conflicts, cleaned up and approved.

Changes made during cleanup:

• Removed unused ProviderDef impl and call_count field from the mock provider
• Extracted run_turn helper to reduce test boilerplate
• Reduced test from 3 turns to 2 (sufficient to prove accumulation)
• Removed assert messages that just restate the assertion
• Removed #[cfg(test)] on inner module (already inside a test module)
• Stripped comment on get_total_token_usage that restated the function name

Note: main already fixed the json/stream-json output paths (using .or() fallback). The remaining unique fix here is get_total_token_usage using accumulated_total_tokens. That function is currently unused but the fix is correct.