fix(annotations): clamp point annotation label x within chart bounds (Fixes #5106)

[waterWang]

Description

Fixes #5106 — Point annotation labels near the left or right chart boundary are cut off when the label text extends beyond the chart area.

Problem

When a point annotation has a long label text and is positioned near the left or right edge of the chart, the label extends beyond the chart's grid bounds and gets clipped (hidden) by the SVG viewport.

Fix

Measure the label text width using before rendering, then clamp the label x position to keep it within the chart's grid boundaries:

• textAnchor = "start" (default): clamp x to [0, gridWidth - labelWidth]
• textAnchor = "end": clamp x to [labelWidth, gridWidth]
• textAnchor = "middle": clamp x to [labelWidth/2, gridWidth - labelWidth/2]

Test

Added 3 new tests in the boundary-clamping describe block:

• Right-edge label: verify x is clamped to less than gridWidth
• Left-edge label: verify x is clamped to >= 0
• Already-fitting label: verify x is > 0