fix(mcp): accept inline args for direct tool calls

[justrach]

Summary

• Closes mcp: direct tool calls drop inline args when arguments is empty #512.
• Adds a direct tools/call regression test for arguments: {} with inline path.
• Normalizes direct call args so non-empty params.arguments remains canonical, while empty/missing arguments can fall back to inline fields or an args compatibility object.
• Updates the empty-args diagnostic so direct calls no longer get a misleading sub-op hint.

Verification

• zig build test -Dtest-filter=issue-512
• zig build test
• python3 scripts/e2e_mcp_test.py --binary zig-out/bin/codedb --project /Users/blackfloofie/codedb-fix-mcp-inline-args

[github-actions]

Benchmark Regression Report

Thresholds: 10.00% and 50,000 ns absolute delta

NOISE means the percentage threshold was exceeded, but the absolute delta was too small to fail CI.

Tool	Base (ns)	Head (ns)	Delta	Abs Delta (ns)	Status
codedb_bundle	73878	71628	-3.05%	-2250	OK
codedb_changes	9085	8736	-3.84%	-349	OK
codedb_context	821968	819892	-0.25%	-2076	OK
codedb_deps	220	201	-8.64%	-19	OK
codedb_edit	37920	39689	+4.67%	+1769	OK
codedb_find	8177	7852	-3.97%	-325	OK
codedb_hot	25274	22195	-12.18%	-3079	OK
codedb_outline	23890	23499	-1.64%	-391	OK
codedb_read	15197	14510	-4.52%	-687	OK
codedb_search	21678	21698	+0.09%	+20	OK
codedb_snapshot	50758	47420	-6.58%	-3338	OK
codedb_status	7717	7894	+2.29%	+177	OK
codedb_symbol	21830	22815	+4.51%	+985	OK
codedb_tree	26334	29504	+12.04%	+3170	NOISE
codedb_word	9702	10981	+13.18%	+1279	NOISE

[github-actions]

Benchmark Regression Report

Thresholds: 10.00% and 50,000 ns absolute delta

NOISE means the percentage threshold was exceeded, but the absolute delta was too small to fail CI.

Tool	Base (ns)	Head (ns)	Delta	Abs Delta (ns)	Status
codedb_bundle	89967	90370	+0.45%	+403	OK
codedb_changes	11117	11453	+3.02%	+336	OK
codedb_context	968616	981419	+1.32%	+12803	OK
codedb_deps	265	258	-2.64%	-7	OK
codedb_edit	50449	51435	+1.95%	+986	OK
codedb_find	10269	10328	+0.57%	+59	OK
codedb_hot	29304	26865	-8.32%	-2439	OK
codedb_outline	28965	31572	+9.00%	+2607	OK
codedb_read	18660	15586	-16.47%	-3074	OK
codedb_search	23492	22429	-4.52%	-1063	OK
codedb_snapshot	64137	59274	-7.58%	-4863	OK
codedb_status	9865	9703	-1.64%	-162	OK
codedb_symbol	19619	17859	-8.97%	-1760	OK
codedb_tree	35138	38443	+9.41%	+3305	OK
codedb_word	11971	13320	+11.27%	+1349	NOISE