feat(seer): Return custom attribute context from get_attribute_names

[DominikB2014]

get_attribute_names dropped user-authored attribute context. Anything that
wasn't a sentry convention or a Sentry-defined field was filtered out before
reaching Seer:

# user-authored custom context is intentionally left out.
if not (context.get("isConvention") or source_by_name.get(name) == "sentry"):
    continue

So custom attributes reached the query agent with no descriptions, even once an
organization had described them.

This routes them into a new custom_fields list, matching the shape in
BROWSE-596:

{
    "fields": {
        "string": ["span.op", "span.description", ...],
        "number": ["span.duration", ...]
    },
    "built_in_fields": [
        {"key": "span.op", "type": "string", "context": {...}},
        {"key": "span.duration", "type": "number", "context": None},
        ...
    ],
    "custom_fields": [
        {"key": "custom_field_1", "type": "string", "context": {...}},
        ...
    ]
}

Notes

• Why a separate list rather than appending to built_in_fields: that list
  means "attributes that are part of Sentry's schema," and its context comes from
  the conventions or a column definition. Custom context is specific to one
  organization, so it travels alongside instead of blurring that distinction.
• built_in_fields is unchanged — conventions and Sentry-defined attributes
  route exactly as before.
• Custom attributes without authored context are unchanged too: they appear
  only in fields, as they always have.

Dependency

Custom context is served by /trace-items/attributes/ via #120994 (BROWSE-595),
which marks it isCustom. Until that lands, custom_fields is always empty —
this PR is safe to merge in either order but only does something once #120994 is
in. It's also gated behind data-browsing-attribute-context, so it stays empty
until an organization authors context.

Seer reads built_in_fields today; picking up custom_fields is a separate
change in the seer repo.

Tests

New unit tests in tests/sentry/seer/assisted_query/test_traces_tools.py
(mocking ApiClient, following test_metrics_tools.py) cover the routing:
custom context → custom_fields, number-typed custom context, Sentry-source
attributes staying in built_in_fields, and no context requested. Verified they
fail if the custom_fields branch is removed.

I also merged #120994 locally and ran the two together to confirm authored
context makes it end-to-end from postgres through the endpoint into
custom_fields — 106 tests passed combined. That merge was reverted; this branch
is off master.

Refs BROWSE-596

[linear-code]

BROWSE-596

[DominikB2014]

@cursor review

[skaasten]

~ I'm surprised these tests are in a snuba api package - seems like they belong in a seer/rpc test namespace?