BigQuery finops: surface bq_region + actionable error messages + warehouse_add warning

[anandgupta42]

Sub-issue split out of #754 to make a focused PR. Covers four of the seven items.

Scope (this issue)

1. Surface bq_region in BigQuery finops tool responses — every finops tool (finops_query_history, finops_analyze_credits, finops_expensive_queries, finops_warehouse_advice, finops_unused_resources, finops_role_grants) returns the queried region as a top-level field on success and error paths. Lets the agent (and humans inspecting tool output) see which region was actually queried after the silent us fallback in fix: BigQuery finops SQL — correct INFORMATION_SCHEMA columns + multi-region support #739.

2. augmentBqError(error, region) helper — appends a region hint to BQ errors that look region-related (anchored to region-<x>.INFORMATION_SCHEMA and Not found: patterns), idempotent, safe on non-region errors. Wired into all five finops catch blocks.

3. isBqPermissionError(error) helper — detects the TABLE_STORAGE 403 pattern that finops_unused_resources hits routinely on project-scoped service accounts, with a word-boundary \b403\b check so unrelated numerics (4031, 40322, port 4030) don't false-positive. Produces an actionable message naming bigquery.resourceAdmin.

4. warehouse_add non-fatal warning — fires when a BigQuery connection is registered without a location field (or with a whitespace-only value). Lists the affected finops tools and tells the user to re-add the connection with "location": "<region>" for non-US projects.

Out of scope (remain in #754)

5. Distinguish "no location provided" vs "location provided but sanitized to empty" — behaviour change with cross-region compliance implications, needs separate consideration.
6. Refactor BQ finops builders to options-object — pure code structure change.
7. E2E test silent-skip warning — test infrastructure improvement.

Reference

Surfaced during the v0.6.1 release review (CTO/PM/Data Engineer/Tech Lead/Chaos Gremlin five-persona panel + 8-model consensus review).