feat: send read-freshness signal on the lance-namespace path

[brendanclement]

Description

db://-style connections that use the lance-namespace path (LanceNamespaceDatabase → NativeTable + the lance-namespace REST client) never sent a read-freshness signal. Against a server configured to serve cached table metadata up to some staleness window, this allows stale-read-after-write across handles and processes. The remote table path already solved this (#3439). This brings the namespace path to parity.

The namespace REST client doesn't let callers attach headers directly, but it forwards a DynamicContextProvider's headers.* context entries as HTTP headers per request. So:

• A shared per-table baseline map is created before the namespace client. I built and installed on the ConnectBuilder via a context provider.
• On read operations the provider emits ·x-lancedb-min-timestamp = max(baseline, now − read_consistency_interval)(RFC3339), keyed by the operation'sobject_id`.
• Each table handle bumps its baseline (monotonically) on checkout_latest(), restore(), and every data/schema write. checkout_latest() is the primary hook: consumers refresh a handle there after writing elsewhere, then poll.

Read operations that carry the floor: describe_table, list_table_versions, query_table, list_tables. list_table_versions is what resolves "latest" for managed-versioning tables (get_latest_version), so it's the op that makes checkout_latest() actually observe a prior write. describe_table_version is excluded (pinned to an immutable version). This mirrors #3439 (timestamp baseline, max(baseline, now − interval), monotonic); no min_version and no body channel, since the namespace path has no version-returning write responses.

Testing

• Unit tests for compute_min_timestamp / next_freshness_baseline and the provider (header at/after a bumped baseline; nothing for an empty baseline + no interval; interval floor applies; non-read ops emit nothing; list_tables uses only the interval floor).
• Verified end-to-end against a local server that honors the header: reads carry x-lancedb-min-timestamp, writes don't, and read-your-write holds.

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[mintlify]

Docs PR opened: lancedb/docs#276

Clarified that read_consistency_interval applies to all db:// remote tables and updated the read-your-write description to match current client behavior.