## Summary
- Adds per-statement query tag support via
`driverctx.NewContextWithQueryTags`, allowing users to attach query tags
to individual SQL statements through context
- Tags are serialized into
`TExecuteStatementReq.ConfOverlay["query_tags"]`, consistent with the
Python
([#736](databricks/databricks-sql-python#736))
and NodeJS
([#339](databricks/databricks-sql-nodejs#339))
connector implementations
- Previously only session-level query tags were supported (set once via
`WithSessionParams` at connection time)

## Usage
```go
ctx := driverctx.NewContextWithQueryTags(context.Background(), map[string]string{
    "team": "data-eng",
    "app":  "etl-pipeline",
})
rows, err := db.QueryContext(ctx, "SELECT * FROM table")
```

## Changes
| File | Description |
|------|-------------|
| `driverctx/ctx.go` | `NewContextWithQueryTags`,
`QueryTagsFromContext`, propagation in `NewContextFromBackground` |
| `query_tags.go` *(new)* | `SerializeQueryTags` — map to wire format
with escaping |
| `connection.go` | Read tags from context → serialize → set
`ConfOverlay["query_tags"]` |
| `driverctx/ctx_test.go` | 5 tests for context helpers |
| `query_tags_test.go` *(new)* | 13 tests for serialization (escaping,
edge cases) |
| `connection_test.go` | 6 integration tests verifying ConfOverlay
behavior |
| `examples/query_tags/main.go` | Updated with session + statement-level
examples |

## Test plan
- [x] Unit tests for `SerializeQueryTags` covering nil, empty,
single/multi tags, escaping of `\`, `:`, `,` in values and keys
- [x] Unit tests for `NewContextWithQueryTags` / `QueryTagsFromContext`
including nil context, missing key, timeout preservation, background
propagation
- [x] Integration tests verifying `ConfOverlay["query_tags"]` is
correctly set (or absent) in captured `TExecuteStatementReq`
- [ ] Verify existing tests still pass (CI)

This pull request was AI-assisted by Isaac.

---------

Signed-off-by: Jooho Yeo <jooho.yeo@databricks.com>
Co-authored-by: Jooho Yeo <jooho.yeo@databricks.com>
Co-authored-by: Vikrant Puppala <vikrant.puppala@databricks.com>