[webserver] Accept user-provided tags on the report_asset_materialization and report_asset_observation REST endpoints

[raghav-reglobe]

Summary & Motivation

DagsterInstance.report_runless_asset_event supports arbitrary event tags via AssetMaterialization(tags=...) / AssetObservation(tags=...) — but the /report_asset_materialization/ and /report_asset_observation/ REST endpoints accept only their allowlisted params and silently drop everything else. External writers in non-Python languages (JVM/Go services reporting events for external assets over REST) therefore cannot attach data-version provenance tags such as dagster/input_data_version/<upstream> / dagster/code_version, which are exactly what makes externally-materialized assets participate in data-version/staleness machinery.

This adds an optional tags parameter to both endpoints:

• JSON body object, or json-encoded query param — mirroring the existing metadata handling (including the 400 on parse failure), plus a 400 when tags is not a json object.
• No new validation surface: tags flow into the existing event construction, where validate_asset_event_tags already exempts system asset-event tags and strict-validates the rest; failures surface through the endpoints' existing 400 path.
• The dedicated data_version param takes precedence over a conflicting dagster/data_version tag (user tags merge first, param applies after).
• ReportAssetMatParam / ReportAssetObsParam gain tags; the materialization API-consistency test's KNOWN_DIFF documents that Pipes does not take tags (same as partition/description).
• Drive-by: fixes a copy-paste typo in the observation handler's construction-error message (it said "Error constructing AssetMaterialization").

Asset checks are intentionally left out — ReportAssetCheckEvalParam has a different shape (severity/passed) and AssetCheckEvaluation has no equivalent tags concept.

Context: we currently run the materialization half as a small runtime patch in production (JVM relays report Iceberg-commit materializations for ~16K external assets with input_data_version tags); upstreaming removes the need to carry it.

Test Plan

Extended dagster_webserver_tests/webserver/test_asset_events.py:

• materialization: tags via json body (system dagster/input_data_version/... + custom key), tags via json-encoded query param, data_version param precedence over a conflicting tag, 400 on non-json query param, 400 on non-object body tags
• observation: tags via json body + data_version precedence, 400 on non-object tags
• both API-consistency tests updated (sample_payload + per-key validation; materialization KNOWN_DIFF)

All 10 tests in the file pass locally.

Changelog

The /report_asset_materialization/ and /report_asset_observation/ REST endpoints now accept an optional tags parameter (json object), allowing runless asset events reported over REST to carry event tags such as data-version provenance — matching the existing Python SDK capability.

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