refactor(helpers): narrow TB reward table ids to str in get_tb_mission_scores

[MarTrepodi]

Description

Unblocks #175 (ty 0.0.56 → 0.0.86), whose Type Check fails on:

error[invalid-argument-type]: Argument to function `sorted` is incorrect
  --> src/swgoh_comlink/helpers/_territory_battle.py:543:26
543 |     if missing := sorted(wanted - curves.keys()):
    |                          ^^^^^^^^^^^^^^^^^^^^^^ Argument type `Any | None` does not satisfy upper bound
    |                                                 `SupportsDunderLT[Any] | SupportsDunderGT[Any]`

The comprehension filtered on one .get() call but yielded a second, un-narrowed one:

-    wanted = {
-        zone.get("encounterRewardTableId")
+    wanted: set[str] = {
+        table_id
         for definition in definitions
         for _, zone in _mission_zones(definition)
-        if zone.get("encounterRewardTableId")
+        if (table_id := zone.get("encounterRewardTableId"))
     }

This is the same walrus idiom the scoring loop below it already uses (if not (table_id := zone.get(...))). Behavior is unchanged, and each zone's id is now looked up once.

Related Issues

Unblocks #175. Once this is in develop, @dependabot rebase on #175 should turn its Type Check green.

Type of Change

• Refactor (code change that neither fixes a bug nor adds a feature)

Checklist

• This PR targets the develop branch
• My commits follow the Conventional Commits / Angular convention
• I have not edited CHANGELOG.md by hand
• Existing unit tests cover the changed path (TestGetTbMissionScores.test_invalid_input_raises, missing-table case (_TB_DEFS, _TB_TABLES[1:]))
• Tests pass (uv run pytest tests/unit: 1511 passed)
• Ruff linter and formatter pass on src/ and tests/
• Type check passes, under both the locked and the incoming ty
• I have not bundled unrelated changes in this PR

Testing

	before	after
uv run ty check src/swgoh_comlink/ (locked ty 0.0.56)	✔	✔
uvx ty@0.0.86 check src/swgoh_comlink/	✖ 1 diagnostic	✔
uv run pytest tests/unit	1511 passed	1511 passed

git merge-tree against #175's head merges cleanly.

Merge danger: two-way door. The blast radius is one internal set comprehension in get_tb_mission_scores, with no public API or output change.

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