[v2 Bug] Full support for --use-v2-parser + Snowflake on mantle

### Is this a new bug in dbt v2.x compared to the latest version of dbt 1.x?

- [x] I believe this is a new bug in dbt v2.x
- [x] I have searched the existing issues and could not find a duplicate

### Current Behavior

I did some digging and found this is already a documented pattern across multiple filed GitHub issues:

- dbt-core#15527 — dbt build with --use-v2-parser on Snowflake fails: Fusion-produced manifest omits adapter macros (list_function_relations_without_caching), so dbt-core's execution phase can't find them. Filed by a dbt Labs engineer.
- dbt-core#15612 — same macro-resolution failure, references fix PR #15529.
- dbt-core#15900 — separate failure mode, also Snowflake: dbt-core can't deserialize the Fusion-produced manifest back (FusionParserSchemaError on the unit_tests field shape).
- dbt-core#15736 — same "Fusion parser failed (exit N)" shape, but on BigQuery — so the instability isn't purely Snowflake-specific.

A customer experienced a crash when attempting to use this flag (Fusion parser failed (exit 1), "Unable to do partial parsing because of a version mismatch"). It doesn't exactly match any single one of the above core issues, but it's clearly the same broadly-fragile surface.

For context, --use-v2-parser was recommended as the short-term mitigation for a gateway timeout in Studio on a large project. With this mitigation itself broken on Snowflake, the customer has no interim workaround until either (a) Snowflake adapter parity lands, or (b) the IDE team's async-timeout fix ships (which only stops the hard-fail, doesn't speed anything up).

### Expected Behavior

Full support/reliability for --use-v2-parser when using Snowflake on Latest Mantle.

### Steps To Reproduce

Customer attempted to run `dbt parse --use-v2-parser` in Studio which returned the error. Environment is on Latest Mantle.

### Relevant log output

```shell
22:31:29 Snowflake adapter: Setting snowflake.connector to ERROR (file logging only)
22:31:29 Snowflake adapter: Setting botocore to ERROR (file logging only)
22:31:29 Snowflake adapter: Setting boto3 to ERROR (file logging only)
22:31:30 Discovered Exposures:
[]
22:31:30 State adapter: dbt-state v2.37.1 is disabled
22:31:30 Delegating parse to v2 parser: /venv/dbt-latest-fusion/bin/dbt parse
22:31:46 v2 parser failed after 16.22s (FusionParserError, exit_code=1)
22:31:46 Encountered an error:
Runtime Error
Fusion parser failed (exit 1); see parser output above.
22:31:46 Resource report: {"command_name": "parse", "command_success": false, "command_wall_clock_time": 19.23911, "process_in_blocks": "0", "process_kernel_time": 2.101696, "process_mem_max_rss": "309940", "process_out_blocks": "99608", "process_user_time": 5.99213}
22:31:46 Observability Metric: command_success=0.0: Runtime Error
Fusion parser failed (exit 1); see parser output above.
22:31:46 Observability Metric: command_wall_clock_time=19.239110946655273
22:31:46 Observability Metric: process_mem_max_rss=309940.0
22:31:46 Command `cli parse` failed at 22:31:46.646228 after 19.24 seconds
```

### Environment

```markdown
- OS:
- CPU: (x86 or ARM)
- dbt distribution and version: (`dbt --version`) v1.12
```

### Which database adapter are you using?

snowflake

### Is this a discrepancy vs. dbt 1.x?

- [ ] Yes — this works in dbt 1.x but not in dbt v2.x

### Additional Context

_No response_